Abstract: Accurate acquisition of the ultrasonic transit time is the most basic requirement for the measurement of axial stresses in bolts. This paper introduced two commonly used methods to obtain the ultrasonic propagation time - threshold method and mutual correlation algorithm, threshold mutual correlation algorithm proposed by the combination of the two algorithms, wavelet noise reduction algorithm supplemented to improve the accuracy of time-of-flight calculation. 8.8 grade M20 × 100 carbon steel bolts were used to verify the reliability of proposed algorithm. The study showed that the threshold cross-correlation algorithm was little affected by the noise signal, and the average error was controlled within 5%, which had good anti-noise performance and good stability, and had greater practicality in engineering environments with large noise.