In order to reduce vibration a new variable structure sliding mode control strategy was proposed in direct-drive air compressor,because torque fluctuate of direct-drive air compressor is large. In order to reduce volume and economize energy, direct-drive air compressor dived by permanent magnet synchronous motor(PMSM). Mathematics model of air compressor torque was built up, and load of air compressor was analyzed. Mathematics model and torque equation of PMSM were built up. Variable structure sliding mode to PMSM was built up. Basing on characteristic to control of PMSM and load of air compressor, the system was simulated in simulink. The result shows that rotate speed fluctuate of direct-drive air compressor controlled by variable structure sliding mode is less than that of direct-drive air compressor controlled by PI. The test of direct-drive air compressor dived by PSMS was done. Experimental results verified that variable structure sliding mode control strategy improve dynamic quality of system and has good robustness. Volume a of Direct-drive air compressor dived by permanent magnet PMSM are little, and its efficiency is high.