In this paper, aimed at the problem of control accuracy when the traditional position-based impedance control is applied in the hydraulic drive unit (HDU) of legged robot, a kind of nonlinear model-based variable impedance parameters controller (MVIPC) is designed. First, the mathematical model of position-based impedance control for HDU is given. Second, the performance of traditional position-based impedance control is tested on the HDU performance test platform under different working conditions, and the experimental results show that the control accuracy of this control method needs to be improved greatly. Thirdly, the control idea of MVIPC is described, and the theoretical derivation is deduced. MVIPC considers the high-order dynamic characteristics of servo valve, pressure-flow nonlinearity of servo valve, oil compressibility and load characteristics. Finally, the control performance of MVIPC is verified on the HDU performance test platform. The experimental results show that MVIPC can significantly improve the performance of traditional position-based impedance control, and have an excellent adaptability under different working conditions. This research can provide an underlying control method of hydraulic systems during the robot locomotion.