For the second time in four years, Ethiopia is facing a major famine. Perhaps three-million peasants could starve.

But outside relief efforts won't help nearly as much as they could until the communist government in Addis Ababa is persuaded to stop using food as a weapon in Ethiopia's civil war.Wisely, President Reagan is trying to do just that. He is putting pressure on Moscow, which keeps the regime of Col. Mengistu Haile Mariam in power by supplying it with weapons and military advisers, to get Addis Ababa to stop trying to starve out the rebels.

In the latest effort to starve the rebels, Mengistu ordered foreign relief workers out of the northern provinces of Eritrea and Tigre, where they had been distributing food contributed from abroad.

The order is bound to hurt drought-stricken farmers and their families more than the rebels, who are well organized and seem able to feed their troops.

If Soviet leader Mikhail Gorbachev wants to persuade the world that he is really as humane and peace-loving as he claims to be, he will heed President Reagan's efforts to get him to have Mengistu lift the expulsion order.

Washington also could stop shipping famine relief to Addis Ababa for distribution. Instead, how about shipping the food to the northern provinces by way of the Sudan?