Abstract: The reasons of abnormal test results because of water path length changes were briefly analyzed. Considering the characteristics of the ring forgings ultrasonic C-scan system, the first interface echo time of ultrasonic flaw detector was used to measure the water path length and the value was further used as a feedback of the actuator for closed loop control. To correct the offset due to mechanical system operation delay and backlash, DAC curve was drawn in real time with spline interpolation technique. For one ring forging with larger than φ1.2 mm equivalent defect, experiments were carried out on the above-mentioned method. The results showed that actual picture quality and the accuracy were better than those that were not corrected. And this paper maybe provides engineering reference value.