Israel would be guaranteed additional aid to pay for U.S. weapons under an unprecedented formula proposed yesterday to the Senate Foreign Relations Committee.

An amendment by Sen. Alan Cranston (D-Calif.) would require giving Israel about $545 million in extra economic aid in the next three years to be used for repaying debts to the United States.

The amendment drew support from several other Democrats but was denounced by Republican members as an open-ended commitment that would considerably escalate U.S. aid to Israel and create demands for similar treatment from other countries buying U.S. arms.

Committee Chairman Charles H. Percy (R-Ill.) called it an "earth-shattering" idea. "It is one of the most astounding proposals I have heard of," he said.

Cranston said that Israel deserves the additional assistance in part because the U.S. policy of arming Israel's Middle East rivals encourages Israel to buy an ever-increasing amount of weapons from this country. "It's our policy that is causing this," he said.

Already the largest recipient of U.S. military and economic aid, Israel makes annual repayments of U.S. arms-sales credits that exceed the amount of economic aid it receives.

The administration is seeking legislation to give Israel $785 million in economic support for fiscal year 1983. In the same year, Israel is expected to repay $910 million in principal and interest on past loans.

Cranston's amendment would require that the United States make up the difference between those sums each year--$125 million in 1983, about $185 million in 1984 and about $235 million in 1985. Each year enough extra economic aid would be supplied to close the gap between arms-sales repayments and the normal economic support funds.

Cranston argued that U.S. policy in the Middle East, especially on arms sales, has contributed heavily to Israel's mounting debt. Israel was advanced $600 million in foreign military sales credits to buy F15 fighter planes to offset sale of electronic surveillance aircraft to Saudi Arabia. Israel also agreed to buy 55 F16s once destined for Iran.

Sen. Charles McC. Mathias (R-Md.) said the amendment would put Israel in a "preferential state" and make its borrowing capacity unlimited. "There would be no limit to the amount of debt Israel could absorb, because someone else is servicing the debt," he said. The committee will continue the debate today.

The panel will also be briefed by State Department officials on the reported suspension of land-reform programs in El Salvador before its scheduled vote on continued military aid to that country.

Meanwhile, in the first indication of congressional dislike of the suspended land reform, the House Foreign Affairs Committee postponed consideration of President Reagan's Caribbean Basin initiative in which the administration proposes to send $128 million in economic aid to El Salvador.

Chairman Clement J. Zablocki (D-Wis.) said the committee wants to hear an evaluation of the land-reform program before taking up the Caribbean plan.