West German Chancellor Helmut Schmidt, in an announcement that surprised delegates at the Social Democratic Party's convention here, said today that he will meet early next year with East Germany's president and Communist Party chief, Erich Honecker.

Although no further details were given, Schmidt's announcement is being viewed here as especially significant because it came just one day before his party is expected to give its approval to a North Atlantic Treaty Organization plan for modernization of Western nuclear arms in Europe, a project bitterly opposed by the Soviet Union.

It also came just a week before NATO is officially to approve the arms decision, which it will combine with an offer to negotiate arms controls in this area with the Soviets who already have deployed large numbers of new missiles and bombers targeted at Western Europe.

Aside from a brief and informal meeting during the 35-nation European Security Conference in Helsinki in 1975, the two top leaders of the divided German nation have never met for official talks.

The last high-level talks between leaders of East and West Germany were nearly a decade ago, when Willy Brandt, then West German chancellor, and Willi Stoph, then East Germany's prime minister, met twice within two months. While the prime minister is the nominal head of government in East Germany, the Communist Party leader actually wields the top authority. e

The apparent willingness of Honecker to meet with Schmidt, a move undoubtedly requiring Kremlin approval, may signal that Moscow is reconciled to the coming NATO decision and is trying to preserve the climate for future arms control discussions even though Soviet Foreign Minister Andrei Gromyko warned publicly in Bonn two weeks ago that the NATO decision could destroy the basis for negotiations.

The announcement is also puzzling, however, because just yesterday Honecker warned West Germany of the "negative consequences" of going along with the NATO decision. Bonn plays a key role because almost half of the planned 572 new U.S.-built missiles would be based here.

Thus, the apparent approval of the meeting, for which no specific date has been announced, may eventually be held as ransom by the East to bring further pressure on Bonn.Or it could be meant as a last-ditch effort by the Kremlin to influence opinion at the political convention here.

Soviet President Leonid Brezhnev said in October that negotiations on the new medium-range nuclear weapons should begin immediately, but only if the West refrained from deploying any. This would leave the Soviets with a considerable edge, Western officials say.

Schmidt's surprise announcement came in a sweeping speech on West German domestic and foreign policy meant to set the tone for next year's federal election campaign here. In the speech he took strong position on two major issues that have caused problems within the party.

The chancellor warned his fellow Social Democrats that West Germany's military security required the West's willingness to maintain a balance of power with the Soviet bloc and that its economic security required nuclear energy.

These two themes -- the NATO arms plan and the cautious reactivation of Bonn's stalled nuclear power program -- are the most controversial within his ruling left-center party and he used the clearest terms he has never used publicly in addressing them.

Schmidt, in effect laid it on the line to his party and those members who are reluctant to go along with what he sees as necessities by suggesting that he could not govern or carry-out policies that he considered wrong.

Schmidt, at 60, is clearly the most politically powerful and respected West German leader since the postwar rule of Konrad Adenauer. He is a good deal more popular than his party, which governs in a coalition with the smaller Free Democrats.

Thus his veiled threat to resign if his party fails to line up behind him reflects his willingness to use his personal prestige to push Bonn in the directions he considers essential.

Schmidt said that certainly there was room for honest compromise on many issues. But he added that the government bore responsibility for what it did and that he, as chancellor, "could not represent any position that I, in good conscience, considered wrong."

Schmidt also declared his support and solidarity for "our American friends and President Carter" in the Iranian crisis, a declaration that drew prolonged applause from the gathering.

Though there is a large minority on the left wing of his party opposed to both nuclear power and new nuclear missiles on West German soil, Schmidt argued forcefully for both and is expected to win majority backing during voting here Wednesday and Thursday.

Schmidt stressed that Bonn remains committed to the interlocked NATO proposal to negotiate controls on these new medium-range weapons. Ideally, he said, negotiations in the installation might eliminate their deployment. Even a result in which fewer than the planned 572 new missiles were deployed would contribute to stability, he said.

He said, however, that NATO could not overlook the continuing Soviet expansion in deployment of similar weapons. He said negotiating with the Kremlin would require no only that the Soviets stop deployment of the multiple-warhead SS20 missiles targeted on Western Europe but that they also remove those missiles already deployed.

Schmidt's party has handled this issue skillfully in a country where there is understandable caution about too much armament.

Former chancellor Willy Brandt, who is still party chairman, articulates the yearnings of the left-wing of the party for disarmament. Schmidt, first cautiously and then more openly, has moved the government toward the more conservative position held by most West Germans.

Yesterday, Brandt described the NATO action as a controversial yet unavoidable decision, while devoting most of his speech to the West German desire for dentente in Europe.

Schmidt said solidarity with the United States and NATO was "the core of our security." Referring to verbal attacks by the east against the expected NATO decision, he warned West Germans not to give in to attempts to put Bonn's loyalty to the test.

On nuclear energy, Schmidt said safety comes first but that Bonn "will not be able to do without the limited development and use of nuclear engergy for the coming decades."

Schmidt's demand for maneuvering room on these is made somewhat easier because he faces a campaign next year against Franz Josef Strauss, the controversial and ultraconservative Bavarian state leader.

Schmidt should win comfortably, but his party is using fear of Strauss as another means of galvanizing party unity, especially from the left.

The Social Democrats, in power here for a decade, have benefited greatly from a triumvirate leadership. It includes the pragmatic and forceful Schmidt, the skillful but aging left-of-center floor leader in parliament, Herbert Wehner, 73, and Brandt, 65, whose respect and ties to the rest of the world rounds out the party's attractiveness to many here and abroad.

But hanging over this meeting is also the question of whether the party has strayed too far from its socialist roots and become primarily a machine to support the more conservative Helmut Schmidt, without whom they could not win in a country where the largest single party is the converative opposition Christian Democrats.