Aiming at the problem that the thread parameters can not be focused correctly in machine vision, the influence of defocus imaging on the thread angle parameters are studied by combining theory and experiment. It is proved that defocus phenomenon can cause nonnegligible error in thread parameter measurement. By analyzing the relation between the sharpness evaluation value of thread image and defocus quantity, a method for correcting the error of thread angle is proposed. Firstly, the sharpness evaluation value and defocus quantity of the image are calibrated, then the sharpness evaluation value of the image is calculated, and the defocus quantity is corrected. Finally, the length and area of the image edge of the selected area are corrected according to the defocus quantity, and the thread angle is calculated using the modified results. The experimental results show that the relative error of the modified thread angle is less than 0.0425% when the defocusing quantity is less than 0.5mm for the screw plug gauge of M20×1.5-6H.