The present invention discloses an ultralow frequency cosine square wave high-voltage generator for distribution cable insulation diagnosis and a distribution cable insulation diagnosis method. The device is formed by a high-voltage direct current voltage source with positive and negative polarities, a dual-directional high-voltage electronic power switch, a polarity conversion device and a total control unit. A test cable is applied by ultralow frequency cosine square wave high voltage with 0.1 Hz period changing having the maximum peak of 30kV. When the waveform is located in the stable states of positive and negative electrodes, the direct-current voltage withstand test of the direct current high voltage to the cable can be simulated to obtain the leakage current of a test object and effectively reflect the centralized defects of the test object, and the alternating current test of the 50Hz alternating current high voltage to the cable can be simulated at an rising edge and a falling edge, and the dielectric loss angle tangent value and partial discharge defects of the test object can be detected so as to obtain comprehensive assessment for the insulation state of the test cable. The total control unit controls the alternating switch of the high-voltage electronic power switch to charge the cable, and the polarity conversion device is employed to discharge the cable to realize the voltage polarity conversion. The ultralow frequency cosine square wave high-voltage generator integrates the cable direct-current voltage withstand test technology and the power frequency alternating current test technology to perform comprehensive integrated assessment of the insulation state of the cable so as to greatly improve the test efficiency, reduce the cost, have the important practical engineering values and improve the industry level.