Partial discharge (PD) magnitude is an important parameter for assessing the insulation performance of cross-linked polyethylene (XLPE) cables. When partial discharges occur in a cable, the propagation of the PD signal through the cable is extremely complex. The traditional cable modelling approach is based on Fourier transform and inverse transform, which is computationally difficult and limited to a specific form of solution. Therefore, this paper proposes a vector-fitting based method for approximating the frequency domain parameters of the cable, which converts the higher-order frequency domain response into the form of a lower-order rational function approximation. The method transforms the cable frequency-variable model into an equivalent model consisting of a cascade of passive element unit circuits by simplifying the transfer function. The tests show that the method can effectively reflect the frequency-variable characteristics of cables and can be used to study the propagation characteristics of partial discharge signals in cables.