In order to assess power cable insulation condition simply and rapidly, this paper proposed a diagnosis method based on the measurements of depolarization current. Insulation conditions of water tree aged samples were assessed using the method, and the diagnosis characteristics parameters of the method were discussed. A water-needle method was employed to obtain water tree aged cables. Using the virtual instrument (Labview), Keithely picoammeters and high-voltage vacuum switch system, a weak current test platform was constructed to collect the depolarization current. Samples of different length were subjected to perform depolarization current measurements, and diagnosis method based on depolarization charge quantity was put forward by integrating the depolarization current measurement curve. The results show that depolarization charge quantity of long cable is greater than that of short one, and the depolarization current values of samples tend to increase with aging time. The experiment indicates that depolarization current measurements can effectively assess the insulation condition of power cable.