The invention provides a transmission line fault diagnosis system based on zero sequence current detection, which belongs to the technical field of power system transmission. In order to solve the problems of the prior art, the system comprises a zero-sequence current transformer, a transmission line passing through the zero-sequence current transformer, a current relay, a collection unit, a transmission unit, a cloud server, a monitoring terminal and a wireless control terminal. The zero-sequence current transformer can capture the fault of a single transmission line. A shielding network canensure that a signal is transmitted without interference in the circuit environment of a load. The current relay can automatically disconnect a failed circuit and send a signal to the collection unit.The collection unit can receive the fault signal of the zero-sequence current transformer of a number of lines at the same time. The transmission unit can transmit the fault signal collected by the collection unit to the cloud server. The cloud server is simultaneously connected with the information of a number of collection units for centralized storing and sends the information to the monitoring terminal, and a worker can find the fault signal given by the zero-sequence current transformer in time.