The reliable operation of high-voltage power cables largely depends on accurately predicting the internal electric field distribution under complex service conditions. In cross-linked polyethylene (XLPE) insulated cables, prolonged direct current or quasi-direct current operation can induce significant thermal gradients and moisture ingress, leading to nonlinear variations in material conductivity and subsequent electric field distortion. This study systematically investigates the steady-state electric field distribution in XLPE cables using multiphysics finite element simulations and numerical analysis. An electric–thermal–moisture coupled model is developed to evaluate the effects of temperature gradient, voltage magnitude, environmental moisture, and the nonlinear conductivity characteristics of XLPE—in particular, the coupled dependencies of conductivity on temperature, electric field, and moisture. At the same time, a MATLAB analysis model is established for error analysis. Results indicate that variations in temperature and moisture significantly influence internal electric field redistribution, particularly under high gradients and strong electric fields. The deviations between the MATLAB fitting results and the COMSOL simulation results are all within 3%, which verified the accuracy and applicability of the proposed physical model. This work provides theoretical support and engineering guidance for insulation design, electric field optimization, and reliability assessment of XLPE cables under electric–thermal–moisture coupling conditions, while ensuring high modeling precision.