In a bid to avert the growing possibility of a political backlash in Washington, Prime Minister Zenko Suzuki's government today decided on an $11.7 billion budget for military spending in 1982, a figure that topped more modest forecasts announced earlier.

Amid mounting calls on Capitol Hill for Japan to bring its present low levels of defense spending more in line with the country's economic clout, Suzuki's Cabinet met at a special session here this evening to endorse plans to increase defense outlays by 7.75 percent next year, compared to 7.6 percent in 1981.

The move, according to government officials and private defense analysts here, was meant as a symbolic gesture designed to demonstrate Japan's good faith in meeting strong, behind-the-scenes pressure by the Reagan administration that it bolster its defensive armor to help offset the burden of American military commitments in the Pacific.

Reagan administration officials have so far shied away from publicly linking trade and defense issues. U.S. congressmen, however, have been less reticent about openly voicing their complaints and officials here are privately concerned about a draft resolution recently introduced in Congress calling for Japan to spend at least 1 percent of its gross national product on national defense.

Japanese Defense Agency officials pointed out that the 1982 defense budget should bring the figure to 0.93 percent of GNP, compared to 0.9 percent in the current year. This is still considerably less than estimates of 5.2 percent for the United States, 3.3 percent for West Germany and 4.9 percent for Britain.

The officials argued, however, that once inflation was taken into account, Japan's defense spending increases next year would come to 4.6 percent, in real terms, which would outstrip the 3 percent increases pledged by the United States' NATO allies.

"This is the best we were able to achieve under current circumstances," said a senior official. Despite the Japanese Cabinet's announcement in June of plans to hold defense spending to 7.5 percent in 1982, Japan's tight-fisted Ministry of Finance has recently insisted that the figure be kept to 6.5 percent in line with its efforts to cut back on Tokyo's massive government deficits.

A senior official at Japan's Defense Agency asserted that the increased appropriations, which now go to the Japanese parliament, the Diet, for final approval, will allow Japan to buy more of the antisubmarine patrol planes, warships and other sophisticated military hardware needed to promote its long-delayed plans to defend the country's air space and sea lanes.

In upping the defense ante, the Suzuki government's chief concern appeared to be what is perceived here as sharply rising U.S. criticism over Japan's yen-pinching military outlays and the record $18 billion surplus it is estimated to have piled up on trade with the United States in 1981.

Relations became badly frayed last December when former defense secretary Harold Brown and other Carter administration officials maintained that they were led to believe that Tokyo would increase defense spending in 1981 by 9.7 percent. The final figure of 7.6 percent led to charges that Japan was shirking its responsibilities.

The Cabinet's 11th-hour decision to boost the 1982 figure to 7.75 percent, a senior defense analyst here said, was made "because if Suzuki didn't do anything, it would look like the Japanese were double-talking again. The figure itself may still look small from the American standpoint, but top Liberal Democrats consider it sufficient as a gesture of good faith."

The defense appropriations represented a departure from those for most other government departments, which were held to an average increase of 6.5 percent under the overall $226 billion government budget for 1982 passed on by the Cabinet today. For the first time in the postwar period, they were allowed to outstrip increases in welfare spending, which are scheduled to be held to 2.8 percent under Tokyo's fiscal austerity drive.

The decision touched off a storm of protest among Japan's socialists, communists and other opposition groups in the Diet. But the political costs to Suzuki and his Liberal Democrats were expected to be slight because of their substantial parliamentary majority and the fact that strong antiwar attitudes among the public have begun to mellow in recent years and now tend to favor a gradual expansion of defense capabilities.

A senior Japanese defense official, whose agency had been insisting on a 7.5 percent military budget increase in 1982, said, "We are really satisfied" with the larger-than-expected appropriation. He said the extra funds will permit the Japanese to place orders for 7 P3C antisubmarine patrol planes, 23 F15 jet fighters, 3 medium-sized destroyers and 12 antitank helicopters along with other new military hardware in 1982.

Japan's military hardware procurement plans, however, still fall considerably short of those envisioned under its current five-year defense buildup program. Under strong pressure from Washington, Tokyo has pledged to complete the program by 1983, one year ahead of schedule.

While critics here remained skeptical that Japan will be able to make good on its promise, Defense Agency officials suggested that additional purchases scheduled for 1983 will bring them within striking distance of the target.

While the Japanese awaited the response of officials in Washington, a spokesman for the American Embassy in Tokyo said, while it was not Reagan administration policy "to discuss specific percentages in relation to Japanese defense spending . . . we are pleased that the budget goes a long way toward implementing the five-year defense plan."