The XLPE cable is widely used in AC / DC power transmission, but its joints and other accessories lack of effective quality assessment means after manufacture, which may lead to potential insulation defects. Therefore, it is of great engineering significance to apply electrical capacitance tomography (ECT) technology to the insulation detection of cable joints. In this paper, the finite element software COMSOL is used to establish the model of cable joint and capacitive sensor, and the insulation detection is investigated by simulation. Firstly, the capacitance is measured to calculate the sensitive field, that is the influence of the spatial distribution of the dielectric constant on the capacitance. It is expected that after the sensitivity field distribution is obtained, LBP algorithm is used to calculate the distribution of dielectric constant, and image reconstructions are successfully realized for two kinds of insulation defects in the cable. In the future, an experimental instrument will be built up to detect the insulation of cable joints.