THE JACKSON-VANIK amendment, linking the grant of equal-tariff privileges to Communist countries to their emigration policies, continues to mess up American foreign policy. Right now, for instance, China, which lets few people emigrate, stands to gain the lower tariffs while the Soviet Union, which is allowing Jews to emigrate on an unprecedented scale, does not. The administration is asking Congress for "most favored nation" status for Peking but is holding back on asking for Moscow. If only China gets MFN, that will mean a conspicuous tilt. The Chinese and American hard-liners will savor it. Those who feel, correctly that the United States should play it down the middle will not.

What's the problem?Some would say it lies in the original conception of the law to link trade and emigration. But some sort of link is surely justifiable as a matter of policy and politics alike. What kind? In 1974 Congress feared that the Soviet Union, the one country then on everyone's mind, would not deliver on emigration if a tight link were not made. Congress (or at least Henry Jackson) also suspected that the executive branch (or at least Henry Kissinger) could not be counted on to keep pushing emigration unless the law let Congress ride close herd. So it was that the law came to demand that countries seeking MFN must offer assurances of future emigration freedom annually. Merely making a good record would not be enough.

This is why low-emigration China is about to get the MFN and high-emigration Russia is not. The Chinese are aware that the American political community couldn't care less, and Deng Xiaoping offered Jimmy Carter 10 million Chinese if he wanted them. But Americans do care for Soviet emigrants, especially Jews, and the Kremlin's leaders simply do not wish to give explicit future assurances - that is, to jump through Sen. Jackson's hoop. The amendment's cosponsor, Charles Vanik, now believes the Soviets have delivered on their part of the bargain and that the Americans should deliver on theirs. Mr. Jackson holds firm.

Congress is right in its concern for emigration. But it is wrong to let that concern be twisted in ways that serve an empty, rhetorical, anti-Soviet foreign policy and that may yet provoke the Kremlin to halt or cut the emigrant flow. If a formula on future emigration assurances cannot be found in the law as it stands - the release of Anatoly Scharansky and action on some of the other highly publicized individual cases might facilitate the search - then the requirement for those assurances should be eased. That would give the administration the flexibility that the requirements of sound overall policy, and of emigration, deserve.