The Japanese government, in a bid to dampen mounting protectionist pressure in the United States before Prime Minister Yasuhiro Nakasone goes to Washington in January for talks with President Reagan, today decided it would cut import duties on a basket of 47 farm commodities and 28 industrial goods.

Tokyo also decided today it would substantially reduce the tariff on imported tobacco -- something which has been a major target of Reagan administation negotiating efforts -- to 20 percent from the current level of 35 percent.

The action was not expected to satisfy the more sweeping demands by American trade officials that Japan move quickly to provide greater access for a broad range of imports, but Japanese officials suggested that further steps may be announced prior to Nakasone's U.S. visit.

Since coming to office in late November, Nakasone has stressed the need to improve relations with Washington. The relationship has been badly strained by economic tensions between the two countries and a chronic U.S. trade deficit with Japan that is expected to hit a record $20 billion this year.

Today's move reflected the new premier's efforts to make greater headway on trade issues against the entrenched interests of the country's powerful bureaucrats, business leaders and elements in his own ruling Liberal Democratic Party. Analysts caution, however, that any truly major concessions are likely to meet with stiff opposition.

Among the 28 industrial goods on the government's tariff-cutting agenda are calculator components, paper products, handkerchiefs, batteries, and other machinery. Import duties on five types of farm tractors, six types of molding machines, and internal combustion engines with more than 500 horsepower are set to be eliminated.

The 40 farm products earmarked for tariff reductions include grapes, raisins, papayas, cotton seed oil, canned peaches, mixed vegetable juice, almonds, avocados, and sugared chewing gum. Today's package is scheduled to be formally announced here Friday after final approval by the governmental Tariff Rate Council. The actual reductions go into effect April 1. They follow a similar round of measures taken on 215 items announced here May 28.

The nuts and bolts of tariff rates, while excruciatingly complex, are important because they are often cited by American businessmen and officials as part of the web of import restraint that keep U.S. products from freely competing in the market here.

The U.S. has charged that high tobacco levies have prevented American producers from expanding their foothold in Japan's lucrative, one billion-dollar cigarette market beyond their current 1.4 percent share. A corresponding cut in internal Japanese taxes could, U.S. industry sources have asserted, bring the price of American-made cigarettes sold here in line with premium Japanese brands.

Reflecting strong domestic political pressures in Japan, however, key members of the Nakasone Cabinet reportedly have stressed that adequate measures would have to be taken to protect the country's 104,000 tobacco farmers. Although tobacco is cultivated in Japan mostly as a sideline crop, the farmers' lobby wields tremendous political clout and the LDP, which relies heavily on the farm vote to stay in power, is wary of tampering with its traditional prerogatives.

Officials said a comprehensive package of market-opening measures, with today's tariff reduction as its centerpiece, would be formally announced Jan. 13. The package is also expected to include expansion of import quotas on six agricultural items, including tomato juice, fruit puree and non-citrus juice.

Tokyo, however, is expected to pass over the full decontrol of beef and citrus fruit sales in Japan, two key commodities for which imports are now restrained by quota. These items have had top priority among Reagan administration officials who view their restriction as symbolic of the closed nature of the Japanese market. But Nakasone's Liberal Democrats have resisted any significant concessions because of a heated campaign by the nation's farmers to block any attempts at further liberalization.