Abstract: Based on the study on semi-active hydraulic suspension model of two-degrees of freedom with fuzzy controller, representing a quarter of a vehicle,a parameters-self-tuning fuzzy PID controller is designed for a half-carmodel of semi-active hydraulic suspensionwith four-degrees-of-freedom. PID parameters can be adjusted on-line according to the fuzzy control rules. Theresults of simulation by using MATLAB/Simulink show that, excited by sinusoidroad input, pulse road input and analogous Grade C road input, the ridecomfort of hydraulic semi-active suspension with fuzzy PID controller isbetter than that of passive suspension and hydraulic semi-active suspensionwith fuzzy controller. Parameters-self- tuning fuzzy PID controllerenjoys better self-adaptation.