China's new relationship with the United States has helped create an unexpected economic and social boom along its strategic southeast coast, laying the groundwork for a breakthrough in relations with anticommunist Taiwan just 10 miles away.

Wen Fushan, vice governor of what for years had been the closed, fear-ridden coastal province of Fujian (Fukien), said here that since 1977 industry has grown more than 6 percent and trade more than doubled in a flurry of port openings and capitalist-style economic experiments. Wen spoke to 24 Peking-based journalists, apparently the first group of foreign reporters to visit this strife-torn province in at least 3 years.

These changes could have a far-reaching psychological impact on the residents of Taiwan, about 80 percent of whom trace their ancestry to, and speak the same dialect as, southern Fujjan. Some officials in the Nationalist Chinese government on Taiwan have already expressed interest privately to visitors in reopening postal links and allowing family visits to the mainland.

Fujian's international ocean trade, after flourishing for 1,000 years, virtually disappeared with the Japanese invasion in the late 1930s. After the Japanese left, a Chinese civil war began immediately. It continued off the Fujian coast long after the Communist took over the mainland in 1949.

Fujian's well-situated port here at Xiamen (Amoy) languished for fear of interference from Taiwan warships and shelling from the Nationalist-held off-shore islands of Quemoy and Matsu. The security-conscious atmosphere smothered the economy and produced some of China's cruelest persecution and bloodiest factional fighting during the late 1960s Cultural Revolution.

But the new era of U.S.-China relations, culminating in full diplomatic ties a year ago, have allowed an end to sporadic shelling off the Fujian coast by both sides and eased fears of any threat to shipping. Peking has suddenly seen a political and economic bonanza in this hilly province with a coast-line full of good harbors.

Fujian has been granted unusual economic autonomy by the pragmatic political successors of the more orthodox late chairman Mao Tse-tung. Fuxhous and Xiamen ports are blossoming with elaborate plans for deep water wharves and "import-export" centers for duty-free processing of imported materials. About 20 docking areas are being opened for small boat traffic up and down the coast.

About 40 percent of Fujian's population of 24 million have relatives living overseas, but these foreign connections have ceased to be grounds for political discrimination here. Religious freedoms for the relatively numerous Buddhists and Christians here have begun to return.

New brick houses and apartment buildings can be seen under construction throughout this provincial capital. Some new buildings in the province are being designed expressly as overseas Chinese retirement homes, or for their relatives here to enjoy. All these developments bear on China's efforts to return Taiwan to mainland control.

If the new economic boom uplifts the still low living standards here to a level closer to that enjoyed on Taiwan, and if the increased religious freedoms and capitalist economic experiments can reduce Taiwanese fears of mainland communism, then some future reunion of Taiwan, and mainland China may be possible, Chinese on both sides of the strait say.

Wen, the 52-year-old vice governor, said, "In ancient times Taiwan used to be a prefecture of Fujian . . . According to some historical records, people started to cross the strait to settle in Taiwan in 230 B.C.

With mutal language, similar costumes and habit and kinshp ties, there has been frequent intercourse between Fujian and Taiwan ever since."

Wen mentioned Fujian's concerted efforts to give refuge to Taiwan fishermen who are shipwrecked or need shelter from storms. He reiterated Peking's year-old offer to let Taiwan maintain its economic system, foreign trade contacts and Army as long as it stops styling itself the Republic of China and instead agrees to identify itself as a local government of the People's Republic.

By small measures, such as allowing Taiwan scientists to participate in international conferences with mainland Chinese, Taiwan authorities have signaled a gradual softening of their hard-line opposition to official contacts with the mainland.

A leading Fujian trade official told reporters at a banquet here that the recent guided tour Taiwan gave to mainland seamen who had been rescued by a Taiwan-bound ship "is a good sign of the interest of the Taiwan authorities in better relations."

Until recently, events in Fujian could not have given a worse impression of mainland life. Fujian residents who later left China now tell stories of teachers and other intellectuals being ruthlessly punished for failure to adhere to Mao's thought and of rival Red Guard youth gangs destroying each other in bloody gun battles and gang rapes.

Asked how many Fujian residents died because of Cultural Revolution violence, Wen said he did not know the total figure. But in the 2 million population Longyan prefecture, he said, where he was at the time, 1,700 people died through suicide, killings or other politically inspired violence.

Fuzhou seems peaceful and stable, with no signs of any recent violence. But the authorities here still seem somewhat uneasy about entertaining foreign journalists. They objected strongly when two reporters who wanted to stroll around the streets tried to excuse themselves from a scheduled visit to a lacquerware factory.

"Fujian Province is not fully opened. We have special programs for you," one official said. Later, however, reporters were allowed to go off on unsupervised walks and even managed to organize an impromptu visit to one of the newly reopened Christian churches here.

The journalists making the unprecedented visit here represent press in Japan, the United States, Britain, Canada, France, West Germany and Yugoslavia. An Australian radio reporter visited here with a delegation from the state of Tazmania two weeks ago.

Authorities here would only sight unspecified "social discontent" for the failure to let journalists enter the province before now. Wen did provide the first official explanation of sorts, for the mysterious 1976 death of Fuxhou military commander Pi Dingjun. Wen said Pi's helicopter crashed in a rainstorm because the pilot's "mind was not at ease due to the influence of the Gang of Four," the Peking Politburo clique that is accused of inciting much of the Fujian violence.

While barring reporters until now, Fjuian authorities have been letting thousands of overseas Chinese come to visit their relatives and recently began to invite foreign businessmen here to discuss trade deals. Officials said last year about 50,000 overseas Chinese and 1,000 foreign businessmen visited.

Aside from the new ports and duty-free processing zones, Fujian factory directors are also being allowed to keep some of their profits for reinvestment in their own plants, a capitalist device that has been allowed only in a few select parts of China.