Abstract: The reciprocating motion of differential cylinder servo system is asymmetrical. In the first part of this paper, a method of nonlinear pressure compen- sation is suggested to compensate this assymmetry. This method eliminates , the influences of area asymmetry of cylinder, variations of load, nonlinearity of discharge and input signal. In the second part of this paper, the transfer function and simplified analysis method of differential cylinder servo system are discussed, the influences of the parameters of area ratio and orters are also analysed.