Currently, cable models used for defect detection in cable grounding systems mostly adopt lumped parameter form, but this model needs to be more accurate in capturing the harmonic characteristics of rapidly changing voltage and current. In order to more accurately capture the harmonic variations caused by defects in the grounding system, a new distributed parameter circuit model for cables is proposed in this paper. The model is based on the cable structure, and this paper details its construction principles and methods, and proposes a decoupling method for three-phase cross-bonded cables. Through simulation experiments and analysis, it is proved that the new distributed parameter circuit model can accurately characterize the signals of the cable core and metal sheath under the normal state of the grounding system, and at the same time, it has the feasibility of detecting the defects of the cable grounding system in the frequency domain.