In order to improve the measurement accuracy of machine vision thread, an evaluation method based on thread image quality was established. Through the analysis of the line gray-level distribution of the thread gray images and the optical imaging characteristics of the thread, the mechanism of the thread image edge distortion caused by the helix angle was revealed. Based on the analysis of the performance of various thread image evaluation methods, the evaluation algorithm L-yakuo based on the thread edge was adopted to calculate the evaluation value of multiple thread images with different object distances. Finally, the experimental comparison and analysis of the M14×2, M20×2.5 thread angle obtained by machine vision and the thread angle obtained by contact measuring instrument were carried out. The experimental results show that after the clearest tooth profile image is obtained by the L-yakuo algorithm, the machine vision thread angle is calculated, and the accuracy of the thread angle of M14×2 and M20×2.5 is improved $9^{\prime }3{3^{\prime }}^{\prime }$ on average. With the help of L-yakuo algorithm, it can sensitively reflect the definition of thread profile, which basically meets the evaluation requirements of thread image definition. The changes of the evaluation value are basically the same as the changes of the relative error of the thread angle, and the evaluation value has the characteristics of high accuracy and easy to calculate.