The device for detecting lack of zero line includes a current transformer. Zero line of three-phase equipment passes through the current transformer. The output end of the current transformer is connected to a light coupling input port. A guard signal for lack of zero line is sent out after the signal process circuit processes the output from the light coupler. Since detecting lack of zero line is implemented by determining whether there is current on zero line without need to know magnitude of current on zero line, the invention overcomes the issue of low precision and resolution of former detection method. Moreover, current transformer with high precision is not needed; or generic current transformer is utilized in the invention so as to lower cost greatly.