The invention discloses a vehicle-mounted wheel set flaw detection method and system. The wheel set flaw detection method comprises the steps that coupling water is sprayed between ultrasonic probes pre-installed on a locomotive and detected portions on wheels under control of wheel set flaw detection signals, the ultrasonic probes are controlled to emit ultrasonic signals and collect ultrasonic signals returned by the detected portions on the wheels, and flaw detection results of the wheels are determined according to the ultrasonic signals collected when the wheels revolve one round. The wheel set flaw detection system comprises a cab unit and a bogie unit; the cab unit comprises an ultrasonic detector and a flaw detection controller, the bogie unit comprises a coupling water spraying unit and an ultrasonic probe assembly, and the ultrasonic probe assembly comprises the ultrasonic probes. The vehicle-mounted wheel set flaw detection system can be pre-installed on the locomotive when a locomotive vehicle leaves a factory and has the advantages that detection is fast and efficient, later line construction is not needed, the flaw detection range is wide, all the portions, needing to be detected, of the wheels can be covered, the cost is lower, and the wheel set health state can be detected at any time in the running process of the locomotive.