Even if the United States were not the most powerful of the West's "Big 7" nations, Ronald Reagan's easy master-of-ceremonies manner would make him the natural center of attraction at the economic summit that opens Sunday.

That's the way it has been since Reagan's first session with the leaders of Canada, Britain, France, Italy, Japan and West Germany in Montebello back in 1981.The new president fought hard at his first summit for "Reaganomics" although his tough monetary policy was a bitter pill for some of his partners. Reagan prevailed.

In 1983, on his home turf at Williamsburg, Va., Reagan ran into a momentary challenge from Canada's Pierre Trudeau, who grumbled about the muscular U.S. foreign policy and said the participants should be working as hard as they can for peace.

But the summiteers wound up endorsing Reagan's plan to deploy new nuclear missiles in Europe. Trudeau backed off, and he won't be at the table to needle the president this time.

In 1985, in Tokyo, Reagan steered the Big 7 into a strong statement against terrorism. Two years later, it is no longer a controversial issue.

One of the few hard decisions expected here is a declaration that governments should not allow airplane hijackers to refuel. The idea is to carry the U.S. no-place-to-hide policy a step further.

The allies, with the exception of Britain's Margaret Thatcher, usually are not that eager to take a militant stand against terrorism. Reagan's biggest accomplishment in his seven economic summits may be lining them up in Tokyo.

But this time it may be a little different. It is Reagan's last round with Thatcher and the other summiteers and he is beginning to take on the mantle of an amiable lame duck.

He will not try to rally his democratic partners to an economic or foreign policy crusade. The agenda is carefully scripted to avoid controversy. And while it cannot be guaranteed to come out that way, the 14th summit of the industrialized democracies will be strong on conviviality and short on decisions.

In a sense, this is a tribute to Reagan's ability to promote his free-enterprise views as well as to America's strength.

Japan and West Germany, bowing to U.S. pressure, are now buying more American goods, and help to Third World debtor nations may take the form of grants or debt forgiving, and not government-subsidized interest rate reductions that the Reagan administration opposes.

For next week's summit, there is an implicit agreement to push the host, Canadian Prime Minister Brian Mulroney, to center stage. Mulroney, facing federal elections in less than 15 months, hopes to profit politically from the summit, and discord would not be the way to demonstrate his skills as a Western leader.

He has pushed divisive foreign policy issues into the background and will try to lead the summit into a quiet discussion of technical economic problems.

At least one could be explosive. The United States would like to bring down agricultural subsidies, and there could be a wrangle with France and West Germany, which are determined to protect their farmers.

But this issue might be sidestepped as requiring further study, and the three-day meeting is apt to be a pleasant closing act on the world stage for Reagan. The biggest concern as the seven leaders head here has nothing to do with economics or foreign policy. It is the plague of our age: terrorism.

The Canadian hosts are taking security precautions. Without the proper credentials, it will be impossible to get within three city blocks of the downtown convention center where the leaders will meet.