The invention discloses derailment detection devices based on bearing saddles positioning and a detection method thereof. The derailment detection devices are installed on side faces, facing wheels, on the bearing saddles and comprise four supporting frames and four laser displacement meters and processor systems respectively installed on the supporting frames. In addition, the invention also provides the detection method. The derailment detection devices and detection method disclosed by the invention realize an early-warning function before derailment of railway vehicles, solve the problem that the derailment of the railway vehicles cannot be predicted, and provide guarantee for ensuring railway transportation safety and reducing freight vehicle derailment accidents.