The probe rotary ultrasonic testing can achieve rapid and automatic detection, and has been widely used by various steel mills. During flaw detection, a water ring is filled between the probe and the workpiece for inspection, and water serves as the coupling medium. It is necessary to solve the problem of rapid stability. The water flow and bubbles can affect the emission and reception signals of ultrasonic waves, leading to a decrease in detection sensitivity, and resulting in defect misdetection and missed detection. In order to improve this phenomenon, an automatic water flow regulation system was designed, which consisted of a programmable logic controller (PLC), an actuator, a flow regulation device, and a flow measurement device. In order to meet the precise regulation of water flow, the system adopted closed-loop control. The control quantity was obtained by inputting the given flow setting value and the deviation value after flow measurement through proportional, integral, and differential (PID) algorithm calculation. It acted on the water flow adjustment device and automatically adjusted the valve opening size, thereby achieving continuous and stable operation of the water supply system, which improved the detection efficiency of the flaw detection equipment and reduced the labor intensity.