The bionic legged robot, driven by hydraulic, has a better ability to operate in different working environment. In order to prevent the robotic foot end from impact and collision during its contact with the ground, it is necessary to design the robotic joint with certain dynamic compliance. Generally speaking, the force based impedance control is a commonly adopted method for active dynamic compliance control in the legged robot's joints. Aiming at the joint driver of legged robot, which is called hydraulic drive unit (HDU), the application of the force-based impedance control method on the HDU is researched. The state space expressions are also developped. The control effect of the impedance control is tested experimentally on the HDU performance test platform. Then, aiming at the main four control parameters, the easier first order matrix sensitivity analysis method is selected according to the advantages and disadvantages of different sensitivity analysis methods. Particularly, the dynamic sensitivity analysis and quantitative sensitivity analysis are conducted for those four parameters under multiple working conditions. Finally, the test experiments are conducted. It is found that the experimental results obtained can be applied under different working conditions. Based on the control performance requirement, the values of control parameters can be compensated specifically and the control strategy can be well optimized.