The utility model discloses an online monitoring and early-warning device of a subway train signal system. The online monitoring and early-warning device is characterized by comprising vehicle-mounted equipment and a maintaining terminal, wherein the data output end of the vehicle-mounted equipment is connected with the data input end of the maintaining terminal; and the vehicle-mounted equipment comprises a central master control unit, as well as a serial processing unit, an auxiliary communication unit, an acquisition input unit and a power supply unit which are connected with the central master control unit respectively. By the online monitoring and early-warning device of the subway train signal system, debugging and maintaining personnel can detect the online operating condition of the equipment in the vehicle-mounted signal system conveniently and the risk of signal equipment failure in the urban mass transit field can be reduced.