VENICE, ITALY -- Ronald Reagan's tightrope act in easing Japanese trade sanctions typified the difficulty of a president in trouble at home trying to lead his peers abroad.

Lifting of what amounted to 17 percent of the punitive super-tariffs against Japanese products was apologetic, amid heckling on Capitol Hill and even in his own administration. Consequently, it was done hesitantly and apologetically. As a boost for Japanese Prime Minister Yasuhiro Nakasone and a demonstration that free trade still lives in the Reagan administration, it fell flat.

If the president could not easily rescind a decision that he probably feels intuitively should not have been made, any stronger leadership seems impossible. Indeed, the mood of Reagan and the leaders of the other six industrial democracies evoked the feeling that they would be happy to leave the city of canals and their annual economic summit with the world no worse off than it was three days earlier.

The president was in no mood to harangue his colleagues. Expected demands that European nations and maybe Japan share the naval burden in the Persian Gulf turned into requests for support of a U.N. cease-fire in the Iraq-Iran war. Instead of putting the heat on West Germany for a more internationally responsible economic policy, U.S. officials praised Bonn for doing what it promises about economic growth -- which is not much. The other six, for their part, laid off harping about U.S. budget deficits.

In this live-and-let-live atmosphere, Japan-bashing was out -- particularly after the lame-duck Nakasone government proposed a $43 billion economic growth package. Thus, before leaving Washington, the president decided on a partial lifting of sanctions. That fit the desires of Secretary of State George Shultz, Secretary of the Treasury James A. Baker III, all governors of the Federal Reserve system and most of the civilized world. Neither the bond market nor the dollar has fully recovered from the original shock of the trade sanctions.

But, the president was advised, it would roil Capitol Hill waters and do no good for passage of a ''moderate'' trade bill. The U.S. trade representative's office and Commerce Department moaned over free-trade backsliding. Commerce Secretary Malcolm Baldrige insisted on language changes, and got them.

It sounded like it. The president's statement was grudging and conditional. If the Japanese do not obey the largely unenforceable semiconductor agreement imposed on them, said the president, ''I will not hesitate to reimpose the partial sanctions that have been lifted.''

If the president lacks enough self-confidence at home to lift these sanctions and so encourage growth-oriented Japanese policies, it naturally follows that he cannot advocate bold economic designs.

A venturesome international monetary reform long ago was ruled out as impossible in today's political climate. But even the modest agreement pledging the seven to observe economic ''indicators,'' set for inclusion in the summit-ending communique', was watered down.

West Germany and Britain flinched at economic targeting. Neither wants the system by which the seven nations could criticize each other for noncompliance. The Germans made clear, meanwhile, they would not stand for a summit statement too vigorously attacking cherished farm subsidies.

Reagan does take these summits seriously, as witness his arrival early -- to avoid jet lag and dominate the news. British Prime Minister Margaret Thatcher (campaigning for re-election) and French President Francois Mitterrand made cameo appearances. Nakasone, more somber than at earlier summits, arrived just before the first session.

But taking Venice seriously is not the same as making it important. It was decided early on at the White House that not much would be decided here. A president who has trouble pulling back from a self-ruinous trade retaliation cannot be taken seriously as leading the industrial democracies to needed economic initiatives.