Faced with runaway growth produced by the past year's ambitious economic reforms and a related spending spree, China's leaders have begun to put on the brakes, according to businessmen and diplomats here.

Indiscriminate spending, combined with uncontrolled bank lending, a sharp drop in foreign exchange reserves and an unrealistically high industrial growth rate, led the country's Communist Party leaders to start tightening control on the economy this spring.

The evidence of that braking is unmistakable now as Chinese bureaucrats confront the many difficulties associated with trying to decentralize a huge planned economy. Party leaders decided recently that 10 of the 14 coastal cities which, together with special economic zones, were to take the lead in China's open-door economic policy, were opening up too fast. They were spending too much foreign exchange on construction and imports with too few exports.

Foreign diplomats and businessmen tend to agree when Chinese officials assert that none of this means a reversal of the country's modernization program. But it clearly signals a temporary slowdown in the scope and pace of some of the country's economic changes.

According to the People's Bank of China, the country's foreign exchange reserves in the six months beginning last September plunged from $16.5 billion to $11.3 billion, a drop of 30 percent.

Huang Wenjun, spokesman for the Ministry of Foreign Economic Relations and Trade, told reporters last week that China's imports jumped 70 percent in the first half of this year while exports dropped slightly in value, mainly due to a decline in international prices for agricultural products.

According to the Far Eastern Economic Review, a Chinese "craze for Japanese cars and television sets" led to a 107 percent increase in imports from Japan in the first six months of the year.

Hardest hit by the slowdown are likely to be exporters in Japan and Hong Kong, businessmen say. But some American businessmen who asked not to be identified said they are being affected. One said that his company, which exports machinery and medicines to China, among other items, already had suffered a drop in business of about 25 percent.

"The Chinese just overspent themselves, and they didn't spend as wisely as people were giving them credit for," the businessman said.

Another American businessman said his company witnessed a "general slowing down of business with China." But, he added, "a lot of negotiations are continuing. A lot of projects are still in the works . . . In the long run, they won't pull back."

Chinese officials said that existing contracts with foreign companies were being honored and that imports of vital importance to the modernization program, including high technology, continued to flow into the country.

In a statement issued recently, the National Council for U.S.-China Trade, a nonprofit organization based in Washington which promotes trade with China, said the current slowdown in 10 cities amounted to a selective and not unexpected "mid-course readjustment."

"This move, while significant, does not signal a major change in China's long-term investment strategy in contrast with indiscriminate, across-the-board retrenchment that took place in 1980-82," the council said.

Although details are still sketchy, the constraints the Chinese government is now imposing on the economy include tighter controls over loans and some contracts with foreign investors. Restrictions on credit and on new construction are likely to slow the industrial growth rate, which according to half-yearly figures released today by the state statistical bureau reveal a jump of 23 percent compared to the same period last year, far above the 8 percent increase planned for the whole year.

Recently, the government also announced that it would impose import taxes of an undetermined magnitude on a wide range of imported vehicles and consumer goods.

Late last year and early this year, Chinese individuals, enterprises, and provincial and city governments began buying television sets, automobiles, tape recorders, and refrigerators at a rate that helped to create a sizable trade deficit. From the Peking leadership's point of view, Chinese banks were lending recklessly and inexperienced enterprises spending indiscriminately.

In the meantime, improvements in China's inadequate energy, transportation and communications systems have not kept pace with its industrial growth. Although electricity production increased 9.1 percent and was higher than last year's growth of 6.6 percent, this still lags far behind the industrial output.

At a press briefing today, Ma An, director of the general affairs department of the statistical bureau, said most of the industrial expansion is taking place in the rural areas and energy shortages are forcing many rural factories to work a three- or four-day week.

Even if the Chinese wanted to import more, they would have difficulty because of congestion in the country's ports. The official China Daily newspaper said more than 500 soldiers have been enlisted in efforts to "unblock" major harbors jammed with ships.

Richard H. Holton, dean of a U.S. team teaching at the National Center for Industrial Science and Technology Management in the Chinese coastal city of Dalian, said in a recent interview: "I think the Chinese started off too quickly. But they've begun to appreciate the need to develop a full technical, social, and managerial infrastructure before they go charging ahead."