Power capacitors are widely used in power system and are the important primary equipment in power system． At present，there is a lack of mature and efficient means to detect the insulation state of power capacitors． So a method based on polarization － depolarization current is proposed to detect the insulation state of power capacitors． The influence of polarization voltage and polarization time in polarization － depolarization current method on the judgment of insulation state is analyzed． The basic principle of polarization － depolarization current method is described，and then two characteristic parameters are deduced，namely DC conductivity and dielectric loss at 0． 1 Hz，which determine the insulation state of capacitors by using the measured polarization － depolarization current． Based on the theory of this method，the polarization and depolarization current curves of power capacitors under different polarization voltage and different depolarization time are studied． The test results show that the polarization and depolarization currents of power capacitors vary under different polarization voltage and time，but DC conductivity and dielectric loss at 0． 1 Hz do not change significantly． It shows that the polarization － depolarization current method can detect the insulation state of power capacitors nondestructively．