THE ANNUAL summit meeting of the seven usually falls somewhere between a serious international conference and a lawn party. The precise mixture depends on circumstances, but in recent years the lawn party aspect of the affair has predominated. Even in the years of least substance, these meetings convey a valuable symbolism. Before a vast audience, they meet -- the leaders of the seven most powerful industrial democracies, natural allies and the principal trustees of the world's prosperity. But each year the question is whether the meeting will actually accomplish anything more than that. As this year's summit convenes in Tokyo, the prospect is better than it has been for some time.

The Reagan administration came to office firm in the opinion that international economic cooperation is little more than an expensive nuisance, interfering in the allegedly pure and serene workings of the market. But over the past year it has moved toward a more active policy and at least tacit recognition that market reactions are often perverse and harmful. It is now dealing with the extraordinary decline of the dollar's exchange rate since early 1985, and the corresponding rise of the yen and most European currencies.

The seven governments can't agree on the ideal exchange rates for their currencies. But they can agree that the rapid movement of recent months needs to be slowed down while trade and industry adjust to it. No govenment, acting alone, can stabilize exchange rates. But these seven, working together, can do a lot to damp down the swings.

There are useful bargains to be struck at Tokyo. Europe and Japan are anxious, and with good reason, about the rise of protectionism in the American Congress. But the resort to protectionism is aggravated by a sense here in Washington that the other industrial countries will do nothing to reduce their own huge trading surpluses, the counterparts of the American trade deficit. If Japan and Germany were to show a serious intention to strengthen their own internal growth -- benefiting their own people as much as the international balance -- they would help President Reagan beat down the kind of mindless and destructive trade legislation that the House Ways and Means Committee has just approved.

Much of the talk at Tokyo will turn, inevitably, to other preoccupations. The Americans want to discuss terrorism. Everybody wants to take up the accident at the Soviet nuclear reactor. Both are subjects well worth pursuing. But each of the seven countries represented depends on foreign trade for an important part of its wealth, and each of them knows that trade trouble abroad quickly becomes economic trouble at home. If they wanted to work together a little more closely to protect their common interests, the present moment would not be a bad time to begin.