HONG KONG, Aug. 27 — After a quarter-century of welcoming and even courting foreign investors, Beijing officials are starting to show considerably more caution. Chinese lawmakers are set to pass legislation this week that would limit foreign acquisitions in China on national security grounds.

Authorities in Washington recently said they would review foreign attempts to purchase American companies working in certain military or technology sectors, including Chinese companies.

The Chinese national security provision is somewhat incongruously tucked into an antimonopoly bill that the National People’s Congress is expected to pass this week, after 13 years of debate and many drafts. Western companies have welcomed many of the law’s provisions, including limits on monopolistic behavior by state-owned enterprises, but the final draft has an unexpected last-minute addition: it states that acquisitions by foreign companies “should go through national security checks.”

The addition reflects what many executives and lawyers describe as greater Chinese skepticism toward foreign investment, even as the Chinese government establishes a $200 billion overseas investment fund and encourages Chinese companies to buy foreign businesses with valuable technology or brands.

“There is certainly a desire by the Chinese government to ensure the crown jewels are not pillaged away by foreign invaders,” said Connie Carnabuci, a partner in the Hong Kong office of Freshfields Bruckhaus Deringer, a multinational law firm.

National security concerns have been raised lately as a reason to delay deals in industries that would not be seen as security risks elsewhere. For example, the Carlyle Group, the American private equity firm, has been trying for nearly three years to buy a construction equipment manufacturer, but government approval is still pending. Even a French purchase of a Chinese cookware company was delayed this year for a national security review; the Commerce Ministry eventually gave its approval.

“Where China used to require foreign capital, and while China used to require foreign managers, now they’re comfortably pat with capital and the managerial class has gotten better, so what do they need foreign investors for?” said John T. Kuzmik, a partner at Baker Botts, a Houston-based energy law firm.

Many experts say that China has long made national security a key consideration in its review of purchases by foreign companies. In this view, the new legislation formalizes that practice.

“This just provides a legal footing for objections,” said John Zhang, a partner at Greenberg Traurig. “In the old days, they probably wouldn’t tell you why they were objecting to a foreign investment.”

The language in the legislation is evocative of American politicians’ complaints two years ago when the state-owned China National Offshore Oil Corporation tried to buy the American oil company Unocal, only to retreat in the face of opposition from Congress.

“Until Unocal, I’m convinced the Chinese really, really believed the United States rhetoric of freer markets and open international borders,” Mr. Kuzmik said. “They were shocked, I believe, at the politicians.”

William D. Watkins, the chief executive of Seagate Technology, said in an interview last Thursday that a Chinese technology company had expressed interest in acquiring an American manufacturer of disk drives. There are only two: Seagate and Western Digital, both based in California. The Chinese government and state-owned media were silent on Monday about Mr. Watkins’s statement.

The new legislation has many provisions sought by Western companies. While earlier drafts, for example, referred to protecting the rights of businesses — rights that could be cited by enterprises seeking to avoid being purchased by multinationals — the latest draft calls for protecting consumers.

Mergers and acquisitions accounted for only 5 percent of foreign direct investment in China before 2004, but rose to 11 percent in 2004 and almost 20 percent in 2005, the official China Daily newspaper reported in its weekend issue. No figure was provided for 2006.

The Chinese government has eased some controls on foreign companies. For instance, Beijing now allows foreign companies to own 100 percent of factories in some industries where joint ventures with Chinese businesses were once required, like auto parts. But in other industries, like car assembly, multinationals are still limited to a 50 percent stake.

“This law I think on the whole is consistent with international best practices — there are some things that are odd,” notably the national security clause, said Jonathan M. Palmer, the managing partner of the Hong Kong office of Heller Ehrman, a multinational law firm.