The invention relates to an ultrasonic internal detector for a pipeline coating. The ultrasonic internal detector mainly comprises a driving mechanism and a rotating mechanism, wherein the driving mechanism mainly comprises a foam body, a steel wire check ring, a deep groove ball bearing, a pin, a straight gear rod, a deep groove ball bearing, a power line, a stepping motor and a detection devicehousing; and the rotating mechanism mainly comprises a sealing water ring, an elastic check ring, a rotating device, a rubber pipe, a probe wire, an ultrasonic probe, a sealing ring, a probe loader, aneedle bearing, an emitting mirror carrying device and the like. The detector is pushed by a medium in a pipe to move forwards, the stepping motor is used for driving, and power is transmitted to therotating device through gear cooperation to guarantee circumferential operation of the rotating device; A water pump transmits a water medium to the rotating device through the rubber pipe, and ultrasonic waves emitted by the probe are transmitted in the water medium and make contact with the pipe wall after being emitted by the emitting mirror. The ultrasonic internal detector can directly detect the damage condition of the coating and can also achieve all-around detection of the pipeline coating through the ultrasonic waves.