The invention discloses an ultrasonic probe module structure for online flaw detection of a train wheel pair. The ultrasonic probe module structure comprises a module seat which is fixed on a train railway and is provided with a mounting hole, and an ultrasonic probe module located above the module seat, wherein the ultrasonic probe module is fixed on the module seat through a pressing plate; the pressing plate is fixed at the mounting hole through a thread connection piece; and the mounting hole is strip-shaped and extends along the length direction of the module seat so that the connection position of the pressing plate and the module seat is adjustable. According to the ultrasonic probe module structure for the online flaw detection of the train wheel pair, the pressing plate is fixed at the mounting hole through the thread connection piece, and the pressing plate and the module seat are detachably connected; the mounting hole is strip-shaped and extends along the length direction of the module seat so that the position of the thread connection piece is adjustable and the connection position of the pressing plate and the module seat is adjustable; and the ultrasonic probe module is fixed on the module seat through the pressing plate so that the position of the ultrasonic probe module on the module seat is adjustable and the position of the ultrasonic probe module is adjustable.