Japan and the United States have agreed on new, simplified technical standards for telecommunications equipment used in homes and offices, which could remove important barriers to foreign companies' sales in Japan, U.S. trade negotiators said today.

In four days of talks in Tokyo, negotiators said, Japan agreed to almost all U.S. requests concerning standards for so-called interconnect equipment, a field that covers a relatively small part of Japan's $25 billion-a-year telecommunications market.

The United States is withholding final judgment until the new rules are published and implemented; but in general, U.S. negotiators were optimistic,, in marked contrast to allegations of bad faith by Japan that have followed other negotiating sessions in recent weeks.

"What we have established is reciprocity of market access at a technical level" in interconnect gear, said U.S. negotiator Jack McDonnel, group vice president of the Electronic Industries Association. "They are not regulating any more than we regulate in the States."

McDonnel called the agreement a "unique achievement" by U.S. trade negotiators. "When they put that on the agenda in December, I thought they were crazy," he told a press conference today.

A Japanese foreign ministry official noted tonight that "everybody is aware of the political sentiment and movement growing on Capitol Hill." He said the changes were in line with Japan's decision to let market forces prevail wherever possible.

The talks concluded an expedited revision Prime Minister Yasuhiro Nakasone promised late last month to Gaston Sigur, a special envoy from President Reagan. Sigur had flown to Tokyo to warn of mounting protectionist sentiment in the United States, which registered a $37 billion trade deficit with Japan in 1984.

In this week's discussions, Japan also agreed to U.S. requests to give equipment suppliers a voice in devising "protocols," the electronic languages by which computers talk to one another across phone lines, and to help set up a private body to oversee the protocols.

Next week, more U.S. negotiators are due in Tokyo to resume talks on trade. U.S officials stressed that other aspects of telecommunications remain unsettled. Today's announcement "does not mean that we will stop negotiating telecommunications with the Japanese government," said team member Clyde Prestowitz, counselor to the secretary of Commerce.

Interconnect equipment includes telephones, facsimile machines, automatic dialers, switchboards and answering machines. The devices generally are owned by the user but are hooked up to telephone lines operated by the phone company.

Standards for such equipment emerged as an important issue late last year, as Japan's Ministry of Posts and Telecommunications began drafting new regulations in preparation for April 1, when Japan's sole phone company, Nippon Telegraph and Telephone, was denationalized and stripped of its monopoloy.

American negotiators said standards were being written in unnecessary detail. "The exact specifications were so detailed that many pieces of equipment could not be sold here" unless costly modifications were made, one industry official said today.

The Japanese wanted standards for such areas as voice quality and strength of the signal sent over the wire. In Japan, the government traditionally has been expected to protect consumers from shoddy equipment, they contended.

U.S. negotiators countered that consumers should make those choices themselves. If parents want a low-quality telephone for a child, for instance, they should be free to buy one. Similarly, new-generation equipment might be barred from use because standards simply did not exist. U.S. officials said the principle for judging equipment should be whether its use would damage the larger network. That principle is in force in the United States.

The regulations that went into effect April 1 contained 30 such standards, but Nakasone agreed to cut them further. In this week's negotiations, Japanese negotiators agreed to eliminate 10, modify two, issue "clarifying circular notices" for six and retain the other 12 unchanged.

Of the 12 to be retained, the United States had pushed for the deletion of three. But U.S. team leader Donald Abelson, of the Office of the U.S. Trade Representative, said that the 10 regulations Japan did agree to drop were the ones the United States found the most objectionable. They included standards on voice quality and signal power.

An account of the outcome of the talks released by Japan's foreign ministry tonight said nine items were to be deleted as well as part of a 10th, otherwise, its account was the same as that of the United States. The changes would be implemented "as soon as possible," Japan said.

The changes should assist sales of certain types of switchboards, digital answering machines and computer couplers, U.S. officials said today.

Today's announcement came as Nakasone convened the first meeting of a special body set up to devise a new market-opening strategy for Japan.