The invention discloses an automatic ultrasonic cleaning device for an underwater building. The automatic ultrasonic cleaning device for the underwater building comprises a distance-measuring sensor, a kernel processor, a fixing rod, a sliding motor, a transducer and an ultrasonic power source, wherein the sliding motor is installed on the fixing rod in a sliding mode, the signal output end of the distance-measuring sensor is connected with the input end of the kernel processor, the distance-measuring sensor is fixedly connected with the transducer and used for collecting the signal of the distance from the distance-measuring sensor to the surface of the underwater building and sending the signal to the kernel processor, the signal output end of the kernel processor is connected with the sliding motor, and the kernel processor is used for receiving the distance signal transmitted by the distance-measuring sensor and generating a corresponding control signal to control the sliding motor to slide relative to the fixing rod. The automatic ultrasonic cleaning device for the underwater building enables the transducer and the surface of the underwater building to keep a proper distance all the time, avoids the phenomenon that the front end of the transducer collides with the building or is clamped in the cleaning process, and achieves the purpose of protecting the device.