The invention relates to a rail vehicle derailment detection method based on a vehicle body contour line. The method comprises the steps of: measuring a distance between a measurement point and a railthrough a range finder installed on a vehicle body underframe, and calculating the lateral displacement of a current vehicle body in a vehicle body coordinate system; measuring the inclination angleof the current vehicle body in the vehicle body coordinate system through an inclination angle sensor on the vehicle body; solving a dynamic outer contour line of the vehicle body in the vehicle bodycoordinate system and converting the dynamic outer contour line into a dynamic outer contour line in a rail coordinate system by combining the vehicle body size, the distribution positions of the measurement points, the transverse displacement of the current vehicle body in the vehicle body coordinate system and the inclination angle of the vehicle body in the vehicle body coordinate system; and dynamically comparing the dynamic outer contour line of the vehicle body in the rail coordinate system with a vehicle judgment envelope line, so that the accidental derailment phenomenon in the vehiclerunning process is detected, and when it is detected that derailment occurs, the system gives an alarm in time and applies a vehicle control action.