Abstract: To clarify the mechanism of insulation degradation in cross-linked polyethylene (XLPE) control cables caused by air gaps, moisture, and the combined defects of both, based on the finite element analysis software COMSOL Multiphysics, this paper systematically studies the characteristics and evolution laws of the electric field distribution of 750 V single-core XLPE control cables under different insulation conditions. The research results show that the defects existing inside the control cable will all cause electric field distortion, but the influence mechanisms and degrees of different types of defects are significantly different. Due to the low dielectric constant, air gap defects mainly cause electric field concentration in the air gap body and its interface. While moisture defects, due to the high dielectric constant of water, cause more intense and larger-scale electric field distortion around the moist area, posing the greatest threat to insulation. When both exist simultaneously to form a composite defect, the electric field distribution shows a nonlinear coupling effect, and the local field strength further increases, highlighting that moisture is the dominant factor inducing insulation degradation.