The three-core cable intermediate joint is a vulnerable component of the cable system.The addition of an explosion-proof box can effectively prevent joint failure from affecting the normal operation of other cables，and also affects the surface temperature distribution of the joint.In this paper，a three-dimensional full-scale model of a 35 kV cable intermediate joint with an explosion-proof box is developed，and a multiphysics coupled simulation model based on finite element theory is established.The influence of contact resistance on the temperature of the joint inside the explosion-proof box and the influence of the gel-filled explosion-proof box on the current-carrying capacity of the joint are studied.By simulating insulation aging and heating defect conditions，the surface temperature distribution characteristics of cable intermediate joints with explosion-proof boxes are obtained.The results show that，due to the thermal resistance effect，the conductor current-carrying capacity of the gel-filled explosion-proof box decreases by 12.77% compared with the bare joint condition，and insulation aging has little effect on the surface temperature rise.When a heating defect occurs at the splice tube，the presence of the gel-filled explosion-proof box causes the maximum temperature region on the joint surface to shift from the traditional end region to the outer surface corresponding to the splice tube.Single-phase and two-phase defects increase the temperature rise by 11.65 ℃ and 13.3 ℃，respectively.Heating defects of cable joints inside gel-filled explosion-proof boxes can be identified by monitoring the temperature rise amplitudes at six positions on the outer surface of the joint at the center of the splice tube.The results provide a theoretical basis for condition monitoring and reliability evaluation of cable intermediate joints and have reference value for optimizing condition-based maintenance procedures for smart grid equipment.