The pressure-compensated plunger pump can better adapt to the extreme deep water environment, but the extreme deep water environment will change the lubrication characteristics of the plunger pair, affecting its underwater working performance and application. Taking the lubrication characteristics of the auxiliary oil film of the pressure compensated pump as the research object, and considering the influence of environmental factors, such as deep water temperature and pressure, on the viscosity, density and other properties of the medium, a numerical model of the auxiliary oil film of the plunger in the deep water environment is established. The parameters that characterize the lubrication features, such as the thickness, pressure, leakage and axial friction of the plunger pair, are obtained by combining coordinate rotation optimization and finite difference discretization in the eccentric direction. The results show that when the water depth is 1 000 m and the rotation angle of the main shaft is about 90°, the auxiliary oil film of the plunger appears in an extremely thin area less than 1 μm, and the pressure difference reaches 80 MPa. When changing the spindle speed and the inclination angle of the swash plate, the leakage and axial friction changes more significantly under air conditions. Under different water depths, there is a positive correlation between the axial friction and the water depth, but the change in leakage is the opposite. This study can provide a useful reference for the design and development of pump plunger pairs in deep water environments.