The tunable damper was designed, then its dynamic performance was analyzed by bench test. The relationship between the damping force and the angle of step-electromotor was developed. It is the foundation for the design of the control system. Based on the simulation results, a fuzzy-controlled semi-active suspension system with the C8051F005 chip microcomputer playing the main role was designed and developed, the bench test and road test for vehicle semi-active suspension were conducted. It shows that the performances of semi-active suspension system is steady and works well. The ride comport of the newly-designed semi-active suspension system is much better than that of traditional passive suspension system. It is a very valuable foundation for the application of semi-suspension system.