The invention relates to a vehicle-mounted OBU abnormal state warning system and method. The system comprises a high-frequency antenna detection module, an OBU state information acquisition module, aninformation processing module and a voice prompt module, wherein the high-frequency antenna detection module is used for detecting whether a high-frequency antenna signal exists in front of a road inreal time or not during vehicle driving process, the OBU state information acquisition module is used for acquiring a working state signal of a vehicle-mounted OBU, the information processing moduleis used for processing and analyzing information provided by the high-frequency antenna detection module and the OBU state information acquisition module, and the voice prompt module is used for broadcasting abnormalitiesof the OBU working state and reminding a driver and passengers of detecting the OBU. According to the method, the vehicle OBU is detected in advance before ETC transaction and path identification of the vehicle, the driver and passengers are noticed with the working state of the OBU in advance, and the OBU is enabled to be a normal working state during vehicle transaction. Bythe system, the transaction success rate of the OBU vehicle can be effectively improved, the passing rate of an ETC lane is improved, and the ETC vehicle path identification success rate is improved.