The intermediate joints of directly buried silicone rubber prefabricated cables are prone to moisture and cause insulation faults. In order to reveal the thermal risk response characteristics under moisture defects, this paper analyzes the local electrothermal characteristics of typical defect areas. COMSOL Multiphysics software was used to establish a three-dimensional cable intermediate joint electricity. Magnetic. In the thermal coupling finite element model, three kinds of spherical particle defects, conductive, semi-conductive and insulating, are introduced respectively, and η is set to represent the degree of spherical particle defects covered by water films of different thicknesses, namely, partial coverage, proper coverage and complete coverage. The electric field and temperature distribution of the defect parts of the cable intermediate joint under different moisture conditions are analyzed, and the simulation results are verified by actual typical faults. The results show that the defect region is easy to form local electric field distortion and Joule heat concentration, resulting in obvious hot spot effect. The conductive defects significantly affect the electric field distortion and local temperature rise in the defect area when η ≤ 1. The insulation defect has the most severe disturbance to the electrothermal field when η > 1. The conductive sphere impurity is most likely to cause electric field distortion and local temperature rise under the condition of η = 1, which is the scene with the highest risk of thermal runaway and needs to be avoided in engineering design. This study reveals the failure mechanism of insulation moisture of cable intermediate joints, which can provide theoretical basis for its structural optimization, moisture-proof seal design and moisture fault diagnosis.