Aiming at the problem of ultrasonic thickness measurement accuracy,this paper analyzes the recognition and calculation method of ultrasonic echo time difference,and proposes a method based on least squares polynomial fitting to extract the peak of correlation coefficient.The method considers the calculation accuracy and computational complexity of the echo time difference,and obtains a more accurate time difference calculation value.The method is applied to the measurement of the thickness of the base metal of the second-line ship lock miter gate of the second phase of Xiangjiang second-class channel,thus the measurement results are accurate and reliable,and provide accurate data support for the quality control of metal structures such as herringbone gates.