The invention discloses a train speed calculation method and system, belonging to the technical field of train control. The method comprises a step of acquiring measurement information of a radar andmeasurement information of a speed sensor and obtaining a speed and a device state of the radar and a speed and a device state of the speed sensor after processing the information, a step of judging the validity of the speeds measured by the radar and the speed sensor according to the device states of the radar and the speed sensor, and a step of combining the speeds of the radar and the speed sensor when the measured speeds are judged to be valid to obtain the speed of a train. According to the method and the system, when a device has a short abnormal state which is a tolerance state and theother two devices work normally, a speed measurement result is correctly provided to ensure the availability of the system.