The invention discloses a novel on-board guarding system. The novel on-board guarding system is mounted in a vehicle body and mainly comprises a power supply module, a controller, a radar module, a Bluetooth unit and a short message module, wherein the radar module is connected with the controller. The Bluetooth unit comprises a system Bluetooth transmitting and receiving module and a key Bluetooth transmitting and receiving module which are matched with each other. The invention further discloses an accident alarming and positioning method based on the novel on-board guarding system. The accident alarming and positioning method based on the novel on-board guarding system comprises the steps of firstly, positioning the position of a vehicle owner by using the Bluetooth unit; if the vehicleis not within a controllable range, starting the radar module to detect whether a mobile object is in the vehicle or not; after detecting an abnormity, starting the Bluetooth unit to detect whether the mobile object in the vehicle is the vehicle owner or not; and if the mobile object in the vehicle is not the vehicle owner, starting the short message module to transmit position information and abnormity information of the vehicle to a vehicle management person. According to the novel on-board guarding system, the motion of an object in the vehicle can be effectively discriminated after parking, an alarm is given in time according to the site conditions to provide assistance to first-time aid.