The invention discloses a testing system and a testing method for the air tightness of a main valve body component of a 120 valve. Leakage between inner air channels is detected through a trace transmitter, and detection accuracy reaches as high as 2.5Pa. Leakage between the inner air channels and the atmosphere is detected through a differential pressure gauge, so that the detection accuracy of the air tightness is greatly improved. A micro-fluidic scheme is adopted for the testing system, and the whole testing process is automatically executed through programs without the influence of human factors, so that the phenomena of erroneous judgment and missing judgment are avoided. A control system and an execution mechanism of the testing system are separated, the control system is universal, and can be jointed with an execution mechanism of a corresponding product to test the air tightness of other products. The testing system is stable in performance, reliable in sealing and convenient to operate, and has a misoperation prevention function.