The utility model discloses an insulating diagnosis of distribution cable is with ultralow frequency cosine square wave high voltage generator. By positive negative polarity high voltage dc potentialsource, two -way high voltage electricity electronic switch, polarity conversion device and always control the unit and constitute. The wave form can be simulated the direct current high voltage and carried out direct current withstand voltage test to the cable when the state is stablized to positive negative pole, can obtain the leakage current of examination article, and effective reaction triesthe centrality defect of article, exchanges the experiment then can simulate 50Hz interchange high voltage to the cable when rising edge and falling edge, detects examination article dielectric losstangent value and partial discharge defect to synthetic aassessment is gained to state of insulation to examination article cable. After charging to the cable through the break -make of always controlling unit control high voltage electricity electronic switch, utilize the polarity conversion device to realize cable reverse charging. The utility model discloses synthesize the experimental method of cable direct current withstand voltage test and industry frequency alternate current, carried out abundant comprehensive aassessment to the cable insulation state.