The invention belongs to the field of electromagnetic ultrasonic nondestructive testing, and discloses an on-line detection device and an on-line detection method for a wheel rim of a train wheel. The invention aims to solve various problems in the conventional detection mode of the wheel rim of the train wheel and to meet the development requirement of a high-speed electric railway. The device consists of an electromagnetic ultrasonic probe array, a preposed case and a controlling and processing unit. The detection device uses electromagnetic ultrasonic vertically incident body waves and uses a pulse reflection method to realize high defect quantification accuracy on the wheel rim of the train wheel. The detection method does not need to use an acoustic coupler or carry out cutting processing on a steel rail. Furthermore, the detection device has simple structure and strong adaptive capacity to environment, and can realize high-efficiency, accurate and overall on-line detection on the defect of the wheel rim of the train wheel.