The invention discloses a GIS AC/DC voltage combined test system and a method. The system comprises a metal shell barrel and an inner connecting guide rod, wherein the two ends of the metal shell barrel are sealed; four separating plates are adopted for separating the metal shell barrel into five air chambers; an alternate current power supply which is composed of an alternate current test transformer and a separating direct current capacitor in serial connection is arranged in the air chamber at one end; the air chamber is connected with a small defective air chamber on central position through an alternate current side witch air chamber; a direct current power supply which is composed of a direct current high-voltage generator and a silicon stack in serial connection is arranged in the air chamber at the other end; the air chamber is connected with the small defective air chamber on central position through a direct current side switch air chamber; the direct current side switch is grounded through an alternate current arrester; the alternate current side switch is grounded through a direct current arrester. The experimental study on the gas insulation combined electrical apparatus insulation defect can be performed in a laboratory under the alternate current, direct current and AC/DC combined voltage. An experimental basis can be established for the accurate detection for the gas insulation combined electrical apparatus insulation fault by adopting different voltage waveforms.