The invention discloses a test system and a test method of a transmission valve. The test system comprises a regulating transformer, two sets of valve units and a signal generator which are in series connection; wherein the regulating transformer is used for outputting testing voltage to the two sets of valve units which are in series connection with the regulating transformer to generate alternating current in the two sets of valve units; the signal generator is used for outputting voltage control signals which are the same in cycle but different in pulse width to the two sets of valve units respectively to generate DC (direct-current) bias current in the two sets of valve units; the two sets of valve units in series connection generate voltage in opposite direction. According to the test system and the test method, during the testing of the valve units, voltages generated by the valve unit to be tested and by the accompanying-tested valve unit cannot be fully offset through the two sets of pulse signals with different pulse widths, and DC voltage component is remained so as to generate the DC bias current in series loop, so that current stress of the valve unit to be tested is the same with that in actual running in the test process.