The direct-drive air compressor drove by permanent magnet brushless D.C. motor (BLDCM) was designed and manufactured. Volume and its weight of direct-drive air compressor drove by permanent magnet BLDCM are little but its efficiency is high. To improve air compressor’s efficiency, BLDCM is used to drive the air compressor. The aim of this paper is to reduce the cost and enhance the efficiency of air compressor, thus based on the direct torque control method, a simple open loop control method to change duty ratio according to the resistant torque was provided. Duty ratios of control cycle beat are different because of the different air compressor load. The duty ratio increase and decrease with the change of air compressor. The system was simulated by Simulink. The simulation results show that the fluctuation of compressor’s rotating speed is small using this control method, and it has robustness.