To avoid external damage to the cable and prevent the spread of intermediate joint faults to adjacent cables. The intermediate cable joint needs to be equipped with an explosion-proof box, but this will prolong the heat conduction path of the intermediate joint, affect its heat dissipation performance, and make it more prone to insulation defects. It is unknown whether the spatial electric field, as an effective insulation defect monitoring method, is suitable for condition-monitoring of cable joints with explosion-proof shells. Therefore, this article studies the spatial electric field characteristics of three core cable joints with explosion-proof enclosures. Firstly, a finite element model for calculating the spatial electric field of the explosion-proof shell three-core cable joint was established, and the spatial electric field distribution characteristics under different defect degrees were calculated. The following conclusion was drawn: as the defect degree increases, the surface electric field of the cable joint gradually increases, and the real-time monitoring of the dynamic increment of the electric field can be used as one of the characteristic parameters for insulation defect monitoring. The dynamic changes in the spatial electric field can reflect the presence or absence of defects and identify the degree of defects. The research results of this article provide theoretical support for the state monitoring of explosion-proof shell cable joints based on spatial electric fields.