The U.S. intelligence community has substantially downgraded the explosive power of the Soviet Union's largest land-based ICBM, the giant SS18.

Latest intelligence estimates on the weapon, included in the classified portion of Defense Secretary Harold Brown's 1980 posture statement, put the destructive force of each of eight or ten warheads carried by the SS18 at about 600 kilotons, equal to 600,000 tons of TNT.Previously, each warhead was carried at about 1.2 megatons - 1.2 millions tons of TNT.

This sharp reassessment of the SS18's yield could play a major role in the coming Senate debate on the U.S.-Soviet strategic arms limitation treaty (SALT II), according to sources on Capitol Hill.

It will bear particularly on the debate about deployment of the MX mobile missile on which President Carter is to make a production decision soon.

The new yield estimate suggests that the SS18 warhead is comparable to the newest generation of American warheads, the Mark 12A, which is soon to be installed on the U.S. arsenal of 300 land-based Minuteman III intercontinental ballistic missiles. The U.S. warhead carries an explosive power of 375 kilotons and is more accurate than the Soviet SS18.

The Mark 12A now is programmed to be fitted on the MX mobile missile that will carry 10 warheads of 375 kilotons each. This means that the warheads carried on the most advanced strategic weapons of both countries will have comparable destructive power, the accuracy of the U.S. missile making up for its lower kiloton range.

Another significant revision in the new estimates was a lowering in the estimated yield of the Soviet Union's second largest missile, the SS19. Its six warheads now are rated at yields of 550 kilotons each instead of the previously estimated 800 kilotons.

All three systems - the SS18, SS19 and MX - will be able to knock out an opponent's ICBM, even in a cement-hardened silo.

Congressional and executive agency sources familiar with the revised intelligence estimates don't agree on the overall meaning of the lower SS18 yield.

A common view, however, is that it illustrates the weaknesses - "uncertainties," as one source put it - of making estimates on Soviet weapons performance that cannot be directly verified.

The Soviet warhead yield, sources said, is deduced from intercepted electronic test data that gives the weight of the missile and warhead carrier.

Using that weight estimate, the intelligence analysts guessed the magnitude of the explosive package in the warheads. Then they estimated the size of the nuclear explosion the Soviets could obtain from the package.

Over a history of more than 20 tests of the SS18 missiles carrying eight or ten warheads, the analysts have successively lowered their estimate of the warhead yield from three megations, to two, to last year's 1.2 and finally the present roughly 600 kilotons.

The sharp change from last year came from a review of past data rather than receipt of any new data from recent Soviet testing.

"Understanding comes gradually," an intelligence source said yesterday, "and uncertainties hang on for a long time."

Partisans in the SALT debate who were aware of the changed SS18 yield were quick yesterday to show how their side of the strategic debate will be helped.

Pro-treaty sources said it indicated more "moderation" on the part of the Soviets and a hint that they are not rushing quite as quickly for a "first strike" capability as SALT opponents have charged. A Soviet "first strike" would be an attack to destroy the U.S. land-based ICBMs in their hardened silos.

Anti-SALT sources said the lowered yield would not "change any view about Soviet intentions" and that the "critical factor was accuracy." In that area, a congressional opponent of the treaty said, the new estimate give the SS18 better accuracy that more than make up for the sharp cut in yield.

Another anti-treaty source, upon learning of the new estimates, shifted from the argument that bigger Soviet megatonnage yields tilted the strategic balance in their favor. This source contended that new estimates of lower yield meant less danger of the Soviets "blinding" their own missiles with the bigger blasts of higher-megaton weapons.

A pro-SALT government official said yesterday the lower yield now projected of the SS18 should have "no effect at all" on the debate.

"We have always said their advantage in megatonnage overall makes no difference," he said. "The fact that we now say it is less than before still does not make a difference."