In order to predict the changing rule of pressure control performance of axial piston pump under oceanic environment, the pressure control system model of piston pump was established based on the underwater hydraulic oil viscosity-stiffness model. The system control performance was comprehensively analyzed from the aspects of stability, rapid response and control accuracy, and the results showed that if the variation factor was the viscosity, the parameters of the system phase margin, amplitude margin, rise time, and steady state were increased from the initial values of 59.4 °, 8.77 dB, 0.045 s, and 3.4% to 138.4 °, 23.4 dB, 0.28 s, and 7.4%, respectively. If the variation factor was stiffness, the parameters of the three characteristics were reduced from initial values to 42.6 °, 23.4 dB, 0.038 s, and 1.2%, respectively.When considering the composite effects of viscosity and stiffness, the parameters of the three characteristics were increased from the initial values to 137.6 °, 23.1 dB, 0.265 s, and 7.3% respectively. The analysis results showed that when the effects of viscosity and considering the comprehensive effects of viscosity-stiffness, the stability of the system increased with the increase of water depth, meanwhile the rapid response and control precision decreased with the increase of water depth. When considering the effects of stiffness, the changing trend of the three characteristics were quite opposite. It was pointed out that the control systems can be regarded as variable viscosity-dynamic stiffness control systems and variable viscosity-fixed stiffness control systems at the sea level of 0~1 000 m and 1 000~7 000 m, respectively. Finally, the experimental results further confirmed the results of the theoretical analysis.
