The invention discloses an on-line wheel tread image detection device for real-time recording of wheel tread conditions; as a wheel rotates, images of wheel tread during one circle of rotation can be taken, so that viewing and subsequent image processing can be facilitated; and during the subsequent image processing, all single images taken by a camera are automatically spliced into a complete image by an image automatic splicing technology. An image detection module can automatically locate and automatically recover the image detection device through an automatic push device. Two light sources are arranged, when a front light source and a side light source act on the wheel tread, uniform illumination is generated, and influence caused by uneven illumination due to an irregular shape of the wheel tread can be overcome. The camera is laid out as follows: the camera is located in the center of the front light source, so that distortion of the taken images is smallest, and the subsequent image processing can be facilitated. The invention also discloses a flaw detection system and a train using the on-line wheel tread image detection device.