Abstract: To address the issue that viscous heating of the oil film during the operation of high-speed hydrostatic motorized spindles leads to a non-uniform temperature field and affects thermal stability and axial rotational accuracy, the distribution characteristics of the oil film temperature field and its influence on axial accuracy were investigated. Based on fluid lubrication theory and the energy conservation equation, a thermal–fluid coupled numerical model incorporating the viscosity–temperature characteristics of the lubricating oil is established to numerically solve the oil film temperature field under high-speed operating conditions. The results reveal a distinct non-uniform axial distribution of the oil film temperature. On this basis, the oil film temperature field model is validated through temperature rise experiments. Combined with thermo-structural coupling analysis and axial rotational accuracy tests, the influence of the non-uniform oil film temperature field on spindle thermal deformation and axial rotational accuracy is further discussed. The simulated results agree well with the experimental results, and the relative errors between the simulated and experimental maximum oil film temperatures are all below 5%, with a maximum relative error of 4.2%. The non-uniformity of the oil film temperature can induce differential thermal deformation of the spindle and change the bearing support state, thereby affecting the axial rotational accuracy of the spindle. The findings provide valuable references for the thermal design and precision control of high-speed hydrostatic motorized spindles.