The invention discloses a partial discharge detection device with joint action of alternating current (AC) and impulse voltage and a testing method thereof. The device comprises a power frequency testtransformer, an impulse voltage generator, an isolation spherical gap, measurement impedance, two filters and an oscilloscope; the power frequency test transformer is connected with a testing sample,the impulse voltage generator is connected with the testing sample through the isolation spherical gap, and the testing sample is grounded through the measurement impedance; two ends of the measurement impedance are connected with the oscilloscope through the filters, and the lengths of signal lines leading out signals at two ends of the measurement impedance are the same. The power frequency voltage and the impulse voltage are jointly acted on a test sample, and the partial discharge signal of the test sample under the power frequency impulse joint voltage action can be acquired by performing interference suppression on a difference value of the partial discharge signals on a measurement ground line and the measurement impedance. The partial discharge performance of the test sample tested by the method disclosed by the invention is more accurate.