After 30 years of stressing bombs and bravado in their campaign for reunification with Taiwan, Communist officials here are adopting different tactics: trade and nostalgia.

For about a year, they quietly have allowed trade with Taiwanese merchants who cross the Formosa Strait in fishing junks laden with radios, watches and umbrellas. Commerce takes place at four specially designated ports along the jagged coastline of Fujian Province less than 100 miles from the Nationalist stronghold on Taiwan.

Although trade only trickles today, it is the first direct and open commercial link between the Communist mainland and the capitalist island in decades. For Chinese on both sides of the strait, it is small but concrete evidence of Peking's commitment to normalize relations with Taipei.

Fuzhou and other parts of this mountainous, seaside province, once known as the war front, have become local testing grounds for the high-sounding policies devised in Peking, 1,700 miles away. Local officials, who have become downright hospitable toward their old Nationalist foes, like to describe Fujian as a political hothouse where the seeds of Chinese reconciliation are taking root.

While a few symbolic gestures are unlikely to lead a Nationalist stampede and in fact have already been denounced in Taipei as "Communist tricks," they take on greater importance since Peking tacitly agreed to link its peaceful reunification efforts to a gradual phase-out of U.S. arms to Taiwan.

In that spirit, Communist authorities in the southern port city of Xiamen have converted underground bomb shelters into cafes and cinemas. Fuzhou radio broadcasters have dropped psychological warfare in favor of nostalgic appeals in programming directed at the breakaway island.

In the farm town of Quanzhou, officials erected a huge, red billboard outside the biggest hotel welcoming returnees from Taiwan and set up a travel agency to facilitate visits.

"We're trying to bring people together," explained Zhang Kehui, vice director of Fujian's reception center for Taiwan compatriots. "The policies will work in time."

To be sure, Fujian is far from becoming a demilitarized zone. Where the frayed port cities give way to a verdant interior of rice paddies and tea groves, the quiet peasant life is disturbed by at least 100,000 soldiers. Seven major airfields sprinkle the province with the best Chinese fighters. And the East China Sea fleet with its 40-odd conventional submarines moors offshore.

Quemoy and Matsu -- Nationalist-held islands off the Fujian coast whose very names evoke American anticommunist rhetoric of the 1950s -- remain on combat alert, bristling with heavy artillery, underground bunkers and about 100,000 of Taiwan's best troops.

Recent foreign visitors to Quemoy were impressed by the siege mentality even as Peking sues for peace. Hunkering down in an observation bunker, a Taiwanese colonel clad in battle fatigues looked across the narrow stretch of water separating him from the mainland and noted that Xiamen was just 2,300 yards away.

"That's less than two miles from communist hell," he advised his guests.

Even the most inveterate communist-hater would concede a change in the outward psychological climate of Fujian from the tense preparedness of just a few years ago. After all, the Chinese side still was lobbing leaflet-filled shells at the offshore islands until 1979.

Local policy began changing when Peking formally shifted from a strategy aimed at "liberating" Taiwan to one of lubricating it with promises and concessions. This new stance became popularized with the slogan "three communications," calling for postal, commercial and navigation links with the island as a first step toward eventual reunification.

When Taipei's response of "no negotiation, no compromise, no contact" came whirling across the strait, the Communist side simply proceeded with its program unilaterally.

Perhaps figuring that one way to the Nationalists' heart is through their pocketbooks, China last fall authorized direct trade with Taiwanese dealers at the four coastal stations on Fujian's 300-mile shoreline.

Vice Director Zhang of the reception center said hundreds of vessels from Taiwan have been lured by the new open door, gliding into the Fujian ports with their Nationalist flags lowered. The traders load their goods into old-fashioned, lugsail junks to slip by Taiwanese customs officials undetected.

Because Taiwan is considered a Chinese province, Fujian authorities allow its goods to enter without customs charges. No money changes hands in the transactions, which involve swaps of Chinese herbal medicines and tea for the island's consumer products.

Zhang was unable to provide trade statistics but suggested that the volume is very small. Another source, the World Economic Herald of Shanghai, estimated that two-way trade through the Fujian stations amounted to about $4.5 million in the first 11 months of 1981.

With Fujian ports now open for duty-free trade, Chinese officials hope they have established a beachhead for direct commercial exchanges and even visits by Taiwanese daring to cross the narrow channel for reunions with their families on the mainland.

Since 1979, several hundred residents of Taiwan have visited Fujian, usually passing through relaxed Chinese checkpoints at the Hong Kong border, according to local officials. Although demand is tiny, provincial travel agencies have popped up in every district.

The reunion theme is now exploited in radio broadcasts from Fujian instead of the old threats and supercharged criticism of "Nationalist bandits." One program serves up what it calls "Hometown News," which gives an account of changes in the villages and describes the lives of their residents.

Another radio show is devoted to "oral letters." Chinese simply go to the radio station, get on the air and name the classmate, cousin or friend in Taiwan they wish to address. Then, they give very personal recollections, usually ending with the stock appeal: "Please, come home."

"The message is always about family matters so as to make the other party [in Taiwan] homesick," explained Xu Zuyi, a Fujian official who helps local people try to recover lost relationships.

Conceding nothing in the war of words, Taiwan has replied with similar broadcasts. When the wind is blowing right, Xiamen residents across the channel from Quemoy can hear loudspeakers blaring: "People of the mainland -- come over to our side."