The invention provides a flaw detection system for an axle. The flaw detection system for the axle, provided by the invention, is used for detecting flaws and damage of the axle. The flaw detection system comprises a top runner device, a probe bracket, a plurality of ultrasonic probes and a data processing device; the top runner device is used for bearing a wheel set installed with the axle to be detected and driving the wheel set to drive the axle to rotate; the plurality of ultrasonic probes are installed on the probe bracket; the probe bracket is used for enabling the plurality of ultrasonic probes to be appressed to the key parts of the axle respectively; when the axle rotates, the ultrasonic probes detect the axle and stimulate corresponding detection signals; and the data processing device is used for analyzing and processing return information of the detection signals and detecting flaws and damage of the axle according to a processing result. The flaw detection system for the axle, disclosed by the invention, is capable of detecting by enabling the plurality of ultrasonic probes to be appressed to the plurality of key parts simultaneously, instead of enabling the ultrasonic probes to be appressed to different positions one by one; therefore, leak detection is avoided; and furthermore, the workload is lower.