THE KILLING of innocent civilians has gone on too long in West Beirut, and it is time for President Reagan to do what he can about it. Until now, he has indulged the Israeli strategists who launched this war. Yesterday, one could see a photograph summing up the results: a seven-month-old baby had lost both arms and been severely burned "when an Israeli jet accidentally hit a Christian residential area in East Beirut during a raid on Palestinian positions to the west." The PLO, meanwhile, holds out.

The Israelis say that before the siege is lifted, PLO forces must be expelled from Lebanon. Israel asserts a right to keep up the military pressure, notwithstanding the risk to civilians, who are advised to evacuate. But are the Israeli terms--flee or take your chances--any less cynical than the PLO's tactic of fighting from civilian cover so as to give pause both to Israel and its patron, the United States? The fact is that Israelis are pounding a city. They have had nearly two months to rout the PLO and they have failed. They alone are responsible for the death inflicted by their guns.

The Israeli way to end the killing, in brief, mandates more killing.

There is, conceivably, another way: for the United States to alter its policy so as to make ending the bloodshed its first aim. The Israelis at least are frank about it: they say it is "imperative" to expel the PLO. Mr. Reagan says it is "imperative" for the bloodshed to cease, but so far he hasn't shown he means it. Even while he calls for a lasting cease-fire, he stands by, doing not much more than shifting from foot to foot, while the Israelis shoot up the city. He has let himself be trapped into thinking that it matters which side violated the latest cease- fire--an exercise with no answers and no ending and one that allows the Israeli army to turn up its firepower any time it pleases.

Mr. Reagan should announce that the United States will use its influence--as much of it as necessary--to see that Israel stops firing into Beirut. The leading Arab states insist they do not need the Israeli army's shooting to help them remove the PLO, and they will have to do the job. No part of the American interest requires Mr. Reagan to save the Begin government from its overextended position in Lebanon, least of all when that makes the United States cosponsor of a cruel attack on an innocent city. Within Israel, after all, not just the little peace movement but main-line opposition politicians and even some members of Mr. Begin's own coalition have questioned his expansion of the aims of the war beyond the original consensus aim of keeping PLO guns from bombarding northern Israel. Why should Mr. Reagan be more supportive of the Beirut siege than, say, former prime minister Yitzhak Rabin?

And if it is really not all that important to Mr. Reagan to halt the bloodshed, then he should drop the pretense and openly shoulder the blame.