The Reagan administration has reversed previous policy and has approved the shipment to China of a sophisticated computer system used in this country for highly accurate simulation of missile flight for the military and the National Aeronautics and Space Administration.

The decision has been controversial inside and outside the government because it comes as the United States is halting computer shipments to the Soviet Union, even down to the level of a computer that plays chess.

Sen. Jake Garn (R-Utah) believes exports to both China and the Soviet Union are dangerous, and is one of numerous sponsors of legislation to stop what Adm. Bobby R. Inman, former CIA deputy director, called "a hemorrhage of the country's technology."

The Commerce Department refused to comment on its approval of the shipment, except to say that trade with China has been greatly liberalized over the past few years, although there is still a prohibition on sending to China "equipment and technology that could make a significant contribution to the design, development or manufacture of new weapons or delivery systems. . . ."

John Celmer, an official in NASA's guidance and control branch, said his agency uses the same type and brand of computer being exported to "do design analysis and simulation of spacecraft control systems . . . to simulate a booster lifting off a pad, the rapid motion of the body" of the missile and what effects control commands for firing rockets have on the motion and direction of the missile.

The computer involved is called a "hybrid" because it combines two kinds of systems. Hybrid computers are used in this country, Celmer said, "mostly in simulations for problems in dynamic vehicles, for space and defense. That's all my experience," he said. But he added that it is possible the hybrid could be used for other kinds of simulation, such as process control in complex chemical plants.

The first part of the $5 million computer system has already been shipped to China. It is manufactured by Electronics Associates of West Long Branch, N.J. The company has tried in previous years to get approval to sell the hybrid computer to China, but was turned down until the Reagan administration's liberalization of trade, according to a company spokesman.

China is now accorded a status above all other communist countries in trade with the United States. But the Chinese, to get the equipment, must say that it will be used for nonmilitary purposes. In this case, the machinery is being shipped to Harbin Institute of Technology, where the hybrid computer will be used in the institute's computer research department.

Electronics Associates says the system will be used for simulation of steel production, chemical processing and turbine dynamics.

Bills to stop such sales have been introduced in the Senate by Sens. Garn, Paul Laxalt (R-Nev.) and Mack Mattingly (R-Ga.) and in the House by Rep. Robin L. Beard (R-Tenn.) and others.