THE JOB of the next secretary of labor is to make the American labor market work more efficiently. That means making it work more humanely, since people who fear change, and regard it as a threat, will fight it. Before the next secretary of labor gets embroiled in all the narrow quarrels over minimum wages, or funding levels of training programs, he needs to think carefully about the larger landscape. There are going to be people in the Reagan administration who regard the Labor Department as irrelevant to the basic strategy. But that's a mistake.

Mr. Reagan, like any president, will want and need steady economic growth. But the process of economic growth, however, admirable in principle, can be very uncomfortable for the people actually caught up in it. Some companies prosper, but others decline as their markets change and fade. The American economy is now coming into a particularly difficult passage, for two of the currently endangered species among its major industries are steel and automobiles -- both carrying enormous political influence, through the corporations that dominate them and through the strongly led unions that represent their work forces.

One way or the other, there are very probably going to be fewer steel and automobile workers in this country four years from now.The companies that compete successfully will have to get their productivity up, and operate with fewer people. The companies that don't compete successfully will disappear. Higher productivity obviously doesn't mean higher unemployment for the economy as a whole. But where people are moving out of high-wage industries like autos and steel, anxieties will be sharp and justified. Perhaps a purist could argue that the blind working of the market is none of the government's business. But common sense replies that the process will go more smoothly and quickly at lower cost, in the presence of a competent and active mediator. That's been the traditional role of the Labor Department.

The greatest pools of unused labor in this country are in those big-city neighborhoods, mostly black, where youngsters grow up amidst endemic unemployment. The Constitution does not require the new administration to build connections between these youngsters and employers. But that would support its supply-side theory of economics, which counts on a sharp increase in productive labor in response to the coming tax cuts.

The next secretary of labor, Raymond M. Donovan is a contractor in New Jersey. The construction business doesn't have much in common with the manufacturing industries. It deals with a family of craft unions that make up a highly special case within the labor movement. Mr. Donovan can hardly have formed views on many of the issues with which he must now deal. But if he seizes the assignment of making labor markets more efficient -- more smooth running, with less reason for anxiety and antagonism -- he will be doing work of central importance to American economic growth.