Abstract: The joints of high-performance quadruped bionic robot are driven by highly integrated hydraulic drive units (HDUs). However, the control of each HDU is difficult, because the loading characteristics of each joint are complex and changeable, furthermore the introduction of the hydraulic transmission intensifies the problems such as the strong nonlinearity and the time-varying parameters. In order to achieve compensation control for trotting gait, the main influence parameters of each HDU are investigated. Firstly, a position servo control system is built, in which dynamic characteristics of servo valve, nonlinear characteristics of pressure-flow, initial displacement of servo cylinder piston and friction nonlinearity are taken into consideration. Then, a simulation model is built based on the structure parameters of HDU and operating parameters. By inputting the friction measured by HDUs, and the displacement and external load measured at each joint in trotting gait into the simulation model, the curves about the displacement and load force of HDU servo control are outputted and verified by experiment. Secondly, sensitivity equations considering the influences of the nonlinearity and time-variant parameters are deduced based on the position servo control equation, and sensitivity function of each parameter with regard to output displacement is also obtained under 4-joint working condition. The maximum value of displacement variation and the sum of displacement variation absolute values in the sampling time are both taken as sensitivity indexes, and their values are shown by histogram. The change laws of each parameter's sensitivity are analyzed. Then, the sensitivity indexes of the four parameters of each HDU are verified by experiment, including supply pressure, proportional gain, initial position of servo cylinder piston and load force. Finally, the primary and secondary influence parameters of position servo control of each joint HDU are obtained for trotting quadruped robot, which provides a reference for position compensation controller design.