A high?precision ultrasonic thickness measurement system based on the least mean square (LMS) adaptive time delay estimation algorithm was developed, addressing the issue of low accuracy of the existing ultrasonic thickness measurement systems. The ultrasonic transmitting and receiving circuits were designed. The high?speed data acquisition and transmission system based on field programmable gate array (FPGA) was developed. The host computer signal processing software based on MATLAB was developed. The ultrasonic time of flight (TOF) was calculated accurately and efficiently by LMS adaptive time delay estimation algorithm, so as to realize high?precision thickness measurement. The simulated echo simulation experiments were conducted. The results show that the LMS method has more advantages in time delay estimation than the peak method, envelope method and correlation method. An ultrasonic thickness measurement system based on LMS adaptive time delay estimation algorithm was built, and the thickness measurement experiments were performed on gauge blocks. The results show that the relative error of thickness measurement is less than 3.77%, the standard deviation of repeated experiment is not higher than 0.2 μm, and the maximum relative measurement uncertainty is 1.4%. The ultrasonic thickness measurement system based on LMS adaptive time delay estimation algorithm can be applied in the fields such as plate thickness measurement, which is conducive to promoting the development of high?precision ultrasonic thickness measurement technology and has important technical reference value.