The polarization － depolarization current ( PDC) method is used to detect the aging state of oil film insulation of surge capacitor． The theoretical basis of the proposed method is introduced，and the testing platform for polarization － depolarization current is built． Polarization － depolarization current of the unaged oil film and the oil film with different degree of aging are measured． The corresponding polarization － depolarization current curve，dielectric loss and DC conductivity are obtained． The causes for the difference in polarization － depolarization current curves are analyzed． The experimental results show that the aging state of oil film can be reasonably detected by analyzing the polarization － depolarization current of oil film of surge capacitor．