There is lack of an effective leakage current monitoring method for a long power cable in a substation. In this paper, based on the analysis on the variation features of leakage current and the imitation of present monitoring methods, a method for a substation AC system based on a combined current criterion with self-compensation is proposed. The residual current and the signal phase current of two ends of the power cable are used. The criterion is a combination of steady-state residual current, residual current increment, steady-state residual current differential, and single-phase residual current differential criteria, to achieve reliable detection on asymmetric leakage and three phase leakage currents of a long cable. The case study results show that the proposed method can monitor different kinds of leakage of a long power cable effectively and promote the level of AC system intelligent maintenance in a substation. This work is supported by the National Natural Science Foundation of China (No. 61803220).