The invention relates to an ultrasonic pipe wall thickness measuring device. The ultrasonic pipe wall thickness measuring device comprises an ultrasonic probe, a reflector, a fixed shaft, a stator, a rotor and a rotor outer shell. The fixed shaft is arranged in the center inside the rotor outer shell, the stator is connected to the fixed shaft in a concentric mode, the rotor is connected to the inner side wall of the rotor outer shell in a concentric mode, the ultrasonic probe is connected to the fixed shaft, the reflector is connected to the rotor outer shell, and an acute angle is formed between the direction of a reflector face and the direction of ultrasonic propagation. According to the ultrasonic pipe wall thickness measuring device, the stator is connected to the fixed shaft, the fixed shaft does not rotate, the rotor is arranged on the rotor outer shell and rotates around the fixed shaft, the rotor rotates evenly and stably after being powered on and drives the reflector to rotate, and ultrasonic waves generated by the probe are vertically reflected to the inner circumferential surface of a pipe wall through the reflector; when the ultrasonic pipe wall thickness measuring device is used underwater, water in the device is centrifugally thrown out through holes formed in the circumferential direction, impact of axial water flow on the reflector face is quickened, the reflector face can be cleaned automatically, therefore, the degree of finish of the reflector face is improved, the ultrasonic waves can not be scattered, and consequently, detection precision is improved.