Mike Mansfield, the U.S. ambassador to Tokyo, yesterday said the United States has achieved 90 percent of its objectives in talks to open the Japanese telecommunications market to American suppliers, and that the remaining 10 percent will be reached within three months.

Mansfield's assessment was the most positive administration evaluation yet of American gains in contentious telecommunications trade negotiations with Tokyo. The issue exploded this month into a test of the United States' relations with its major Pacific ally and sparked overwhelming House and Senate votes urging trade retaliation against the Japanese.

Other administration trade officials, key congressmen and industry representatives remain skeptical over the actual effect of Japan's efforts, however. They argue that U.S. access to the Japanese market depends to a large extent on how fairly the new regulations are applied by the powerful Japanese bureaucracy, which traditionally favors domestic suppliers.

Mansfield, who has been the U.S. ambassador to Tokyo for eight years, tempered his praise for Japan's liberalizing moves in telecommunications with a warning that Tokyo has to continue to allow foreign companies greater access to its markets in all areas.

The cry with Japan should be "access, access, access -- like football fans yell 'defense, defense, defense' ," Mansfield said in an interview with The Washington Post.

"We want them to open their markets in their own best interests. They are the major beneficiaries of a free-world trading system and will be hurt the worst if it breaks down," Mansfield said.

Mansfield, who met twice this week with President Reagan over the current crisis in U.S.-Japanese trade relations, expressed concern in the interview over the increasingly emotional congressional attacks on what is seen on Capitol Hill as Japan's protectionist trade policies, and said he hopes "sanity and reason will take over" during the Easter recess. The U.S.-Japan relationship, he said, "is too important to be disrupted by a wave of emotionalism.

"The relationship is too valuable, too strong, too precious to let differences of the moment create a situation which we will be sorry for in the future," he said later.

While an anti-Japanese feeling "exploded" in Congress, Mansfield said, "I'm not at all certain it exploded in the country."

Mansfield said he is "disturbed at the way Japan is being made the scapegoat" for the United States' $123.3 billion trade deficit, $36.8 billion of which resulted from trade with Japan. Mansfield urged Congress and the administration to work together to solve the country's own economic problems -- the overvalued dollar, high interest rates and soaring budget deficits -- that bear the major responsibility for the trade deficit.

"The Japanese have to open their markets. We have to do our thing here," he said.

But Mansfield, 82, a Democratic senator from Montana for 25 years before turning diplomat, credited the strong attacks on Japanese trade practices by legislators noted for their free-trade attitudes, such as Sen. John Chafee (R-R.I.), with helping speed the trade talks.

He said Chafee's introduction of a bill that would bar Japanese telecommunications products from the United States if U.S. companies didn't get equal access in Japan "the straw that broke the camel's back" during the Tokyo negotiations.

"It probably had a large part to play in bringing the negotiations to a successful conclusion," Mansfield said.

The government of Prime Minister Yasuhiro Nakasone was so shocked by the new anti-Japanese mood of Congress that it rushed a special emissary, Deputy Foreign Minister Reishi Teshimi, to Washington to explain Japan's position to Congress and administration officials. Teshimi had trouble meeting with congressional leaders Thursday and yesterday because most of them had left the city for the Easter recess, but he had a busy schedule of administration appointments.

Mansfield said Nakasone took great political risks to gain the trade concessions the United States demanded.

"We should appreciate his difficulties and the [domestic] problems he has to contend with," the ambassador said, adding that Nakasone has more trouble with rival factions within his ruling Liberal Democratic Party than with opposition politicians.

The list of recent Japanese trade liberalization moves, furthermore, go beyond telecommunications to financial markets and tobacco sales, areas where the Reagan administration has pushed for action, Mansfield said. Japan, acting under intense U.S. pressure, also agreed yesterday to drydock its commercial whaling fleet after 1988.

He was optimistic about market-opening initiatives in three other areas targeted by Reagan and Nakasone during their January meeting in Los Angeles. There is "a good chance" Japan will buy American-made communication satellites; "progress" has been made in initial talks to gain more U.S. sales in sophisticated electronics and pharmaceuticals and medical equipment, and "we finally have been able to break the stone wall" erected around Japan's politically potent and highly protected lumber and wood products industry, Mansfield said.

Even though it will take two to three years under Nakasone's plan to reduce trade barriers in lumber, Mansfield said, "it is the first sign of light at the end of the proverbial tunnel we've seen in all the years I've been in Japan."

Over all, he said, "The Japanese market is not as closed as many Americans think, and ours is not as open as many people say."