The invention discloses a traction network impedance on-line testing device and a test method, belonging to the electric railway traction power supply technology field and solving the on-line test of the traction short circuit impedance. One end of an isolation switch K1 of a current generator CG1 is connected to one end of an AC port of a PWM transformer through a current mutual inductor CT1; a voltage mutual inductor PT1 is connected to an AD1 port of a PWM current transformer in parallel; the measurement terminals of the voltage mutual inductor PT1 and the current mutual inductor CT1 and the control terminal of the PWM current transformer are connected to the controller CU1; the controller CU1 is connected to the Internet IT; a pantograph EB of a current generator 2 CG2, an isolation switch K2, and a current mutual inductor CT2 are connected to one end of the PWM current transformer AD2 after serial connection; the voltage mutual inductor PT2 is connected to the AC terminal of the PWM current transformer in parallel; the measurement terminal of the voltage mutual inductor PT2, the measurement terminal of the current mutual inductor CT2, and the control terminal of the PWM current transformer AD2 are connected to the controller CU2; and the controller CU2 is connected to the Internet IT. The invention is mainly used for the traction network impedance on-line test.