BEIJING—A meeting of nearly 3,000 lawmakers in Beijing is set to test the clout of Chinese President Xi Jinping, after he placed allies in key government posts over recent months in a bid to steer a pivotal leadership shuffle this fall.

This year’s session of the National People’s Congress, which starts Sunday, marks the last major political event before the Communist Party elite gathers to set China’s power structure for the next five years.

Near the end of his first five-year term, Mr. Xi faces no clear challenger, having used disciplinary campaigns to neuter rivals and demand fealty from the party’s 89 million members. Some party insiders say Mr. Xi may be maneuvering to stay in power beyond a second five-year term, which would be a break from recent practice.

But resentment has been brewing over Mr. Xi’s antigraft campaign, his attacks on vested interests in government, state-owned industry and the military, and slow progress on overhauls to China’s growth model.

The NPC is a largely rubber-stamp parliament but one where delegates have in the past criticized government policy. Observers expect Mr. Xi to try to curb any airing of grievances, as any political controversy now could jeopardize his success in promoting allies and sidelining rivals in the fall party conclave.

Mr. Xi “is more interested than ever in showcasing the party’s unity and its achievements,” said Matthias Stepan, a specialist in Chinese domestic politics at the Berlin-based Mercator Institute for China Studies.

The annual parliamentary meeting follows the appointment of more than 130 senior provincial-level officials to new positions in January and February, well above comparable tallies from recent years, according to government and state-media notices.

A high-level shake-up began late last year when two officials seen as close to Mr. Xi were named as state-security minister and Beijing mayor. Last week, two others were promoted as China’s commerce minister and top economic-planning official. Some of Mr. Xi’s associates gained posts seen as steppingstones into high-powered roles in the party leadership.

The changes have coincided with a rising state-media drumbeat lauding Mr. Xi’s signature programs, including a poverty-alleviation campaign and the four-year drive to root out corruption.

On Thursday, the party’s official mouthpiece People’s Daily released a video titled “People’s Representative Xi Jinping,” lauding him as a strong statesman who endured personal hardship and understands the needs and aspirations of ordinary Chinese.

In recent weeks, Mr. Xi has stepped up his campaign to enforce “strict party governance,” urging officials to demonstrate “self-discipline.” The party’s top disciplinary agency this week disclosed a number of actions against officials across China for failing in their management responsibilities.

For the NPC—a largely ceremonial affair in any year—such efforts mean divisive topics are likely to stay off the agenda. This week, for example, China’s labor minister signaled an indefinite delay to a proposal to raise the statutory retirement age, despite warnings from economists that Beijing must urgently address demographic pressures from an aging population. The minister, Yin Weimin, had previously pledged to complete a plan this year, but on Wednesday he said the issue has stirred “very heated” public debate and that a proposal will be rolled out “at the appropriate time.”

NPC delegates are expected to discuss, among economic and social issues, Mr. Xi’s “thoughts on governance,” with emphasis on his authority as the party’s “core” leader, according to the official Xinhua News Agency.

That doesn’t mean power struggles won’t play out behind closed doors. Party infighting spilled into the open at the 2012 meeting, when then-Premier Wen Jiabao issued a veiled rebuke of Bo Xilai, a regional party chief and Xi rival, in a harbinger of Mr. Bo’s downfall.

Straying from the script could be costly. Any loose comments perceived as critical of Mr. Xi could invite reprisals, said Steve Tsang, director of SOAS China Institute in London. “No one will want to say anything silly at the moment.”

Last year’s expulsion of 45 NPC delegates from northeastern Liaoning province struck a nerve. Their alleged offense was electoral fraud, but other lawmakers saw the episode as an implicit threat against dissenters. “Liaoning was taken to task to set an example,” said an Asian diplomat based in Beijing.

In lieu of serious policy debate, observers say this year’s NPC offers a chance to scrutinize newly appointed officials for their fitness for higher leadership roles.

At the party conclave this fall, up to five of the seven current members of the Politburo Standing Committee—the top leadership body—are due to step down, if existing retirement norms hold. In addition, more than 60% of the 376-seat Central Committee—which includes ministers, state industry chiefs and army generals—are expected to be replaced.

Party insiders say Mr. Xi is well placed to fill these vacancies with loyalists, but he must still vie with departing and retired leaders who seek to promote their own favorites.

Speculation is rife among party insiders and political observers that Mr. Xi will try to break from recent retirement norms for top leaders, which currently bar them from starting new five-year terms at 68 or older. If Mr. Xi succeeds, analysts say a beneficiary could be Wang Qishan, the party’s top disciplinarian and Standing Committee member, who turns 69 in July.