Former secretary of State Henry Kissinger said today that the Reagan administration erred last week when it offered to sell substantial quantities of grain to the Soviet Union without trying to extract political concessions from Moscow.

Kissinger said that the offer of an additional 15 million metric tons of U.S. grain also gives Europe more ammunition for its charge that the U.S. is hypocritical when it penalizes European companies that sell gas pipeline equipment to the Soviet Union at the same time it encourages American farmers to sell grain to the U.S.S.R.

He told the annual meeting of the American Bankers Association here, however, that the gas pipeline would be built no matter what the U.S. posture on its own trade with the Soviet Union.

Kissinger, now a consultant on international affairs, said the West must increase its use of trade as a lever to get the Soviet Union to agree to such changes in policy as mutual arms reduction, abandonment of support for terrorist groups, use of "proxy" troops in faraway areas like Africa and a commitment not to use its own army outside its borders.

The former secretary of state conceded that some level of grain sales is mutually beneficial to both Moscow and Washington. The United States and the Soviet Union have agreed that Moscow will buy at least six million and as much as eight million tons of grain a year. Last week, in an attempt to bolster sagging U.S. grain prices, the administration announced that it would be willing to sell up to 23 million tons to Moscow.

President Reagan also guaranteed that grain bought within the next six months would not be subject to any embargo the administration might find necessary to impose in the future.

Reagan has been criticized by leading Democrats for using the grain sales issue as a political ploy three weeks before the November elections. He has been criticized by some farm groups for now removing all restrictions on grain sales to the Soviet Union.

Kissinger, in his speech today, sidestepped both those issues.

But, he said, by refusing to extract some political concesions from the U.S.S.R. in return for the grain Moscow needs so much, the administration is "giving away one of our assets too cheaply."

He said the Soviet economy is in such terrible shape that the leadership can put "no more pressure on [its] consumers without returning to a police state," a reversion the Soviet system presumably could not tolerate.

Instead, Kissinger said, careful use of the trade weapon could force the Soviet Union into a reform of both its internal and external policies. U.S. problems, he said, are solvable. The difficulties the Soviets face are "systemic," Kissinger said.

By sharply boosting grain exports, Kissinger contended, the United States strengthens the Soviet capability "to solve its domestic problems" without facing the questions of what it must do about its heavy military spending, its stagnating consumer and farm economies and its aging leadership.

The United States and its Western allies must reach an agreement on what items are appropriate for purely economic trade with the U.S.S.R., what credits and concessions should be granted on exports to the Soviet Union and how Western trade policies with the East can be made homogeneous, he said. "If we stick to or guns, there is an opportunity for a fundamental change in East-West relationships."