This paper analyzes the current characteristics of short circuit and open circuit of single thyristor as well as the current characteristics of open circuit of AC line, and comes up with a new on-line fault diagnosis method for the rectifier device. For the single thyristor short circuit, the theoretical DC value calculated using the measured AC current will be greater than the real-time DC value measured, and the DC components of the fault phase and non-fault phase are opposite in sign, which can be used to locate the fault thyristor. In the open circuit situation, if the DC component value of current calculated by slide window is continually larger than the threshold value, it is single thyristor open circuit, and the fault thyristor can be identified by the sign of fault phase DC component. If the DC component value of current firstly increases above a certain value and then reduces to 0, the fault can be identified as AC line open circuit, and the fault phase can be located with the help of minimum amplitude of the fundamental wave. Simulation results show that the method can diagnose the faults quickly and accurately and that it can also be applied to the fault diagnosis of multiple parallel rectifiers. So it has theoretical and practical value. This work is supported by National Natural Science Foundation of China (No. 51177128).