VENICE IS A MAGICAL city, but perhaps not the best place in the world to discuss the somber subject of economics and finance. There are too many distractions, a thought that has probably already occurred to the seven governments that gather there today. It is supposed to be their annual summit meeting on economic policy, but most of the seven people who lead those governments have other and more pressing things on their minds at the moment.

President Reagan is focused, first of all, on the Persian Gulf and his efforts to get the Europeans to provide more active help in guarding sea lanes. His second concern is arms control, and the intricate diplomacy to maintain agreement among his European allies on the negotiations with the Russians. As frequently happens in this administration, the economic agenda ranks at best third in its considerations.

The British and Italians are preoccupied with their elections. The British vote Thursday, the day after the Venice meeting ends, and Prime Minister Margaret Thatcher greatly upset the Italians by announcing her intention to arrive late and leave early. Italy's election is next Sunday, and one of the few certainties is that the next prime minister won't be the present caretaker. As for the Germans, they are interested in economic coordination only in the negative sense that they wish to avoid the subject as much as possible. They fear it will mean -- as, of course, it should -- an attempt by everyone else to press West Germany to speed up its own growth.

The conference may not choose to deal with that directly, but that's the main issue before it. The world's economy has been growing more or less steadily for nearly five years since the last recession, and that's rather a long cycle. Without careful management, it will shortly slide into another recession. The Americans waited far too long to start reducing their budget deficit, but this year it's going to drop sharply. The other industrial democracies, accustomed to depending on soaring exports to the United States, are going to see the American market suddenly weaken. The Japanese government has noted this danger and is beginning to strengthen its domestic demand to compensate. But the Germans, fixed on their fears of inflation, refuse to move, and West Germany will determine the pace for all of Western Europe.

Recessions can't be prevented. But they can be mitigated and deferred. That's the job that the seven eminent politicians at Venice have before them. They have many other preoccupations at the moment, most of them more immediately compelling than the tedious business of economic fire prevention. But this year's economics is next year's politics