In order to improve the accuracy of radius measurement of round hole parts, an improved Zernike moment sub-pixel measurement method of round hole parts is proposed. First of all, the traditional Canny operator is improved, and the image input, denoising, gradient amplitude calculation and threshold selection are optimized to realize the rough location of the pixel-level edge coordinates of the circular hole center. Secondly, the edge pixel of the target region where the round hole is located is extracted, the new edge judgment condition is put forward on the basis of Ghosal algorithm and the best gray step threshold of Zernike moment is calculated by iterative method to judge and obtain the sub-pixel edge point, and the error is analyzed from the point of view of gray edge model. Finally, the high precision detection and measurement of the center coordinate and radius of the round hole are realized by using the least square principle. The simulation results show that the relative error of the center coordinate of the improved algorithm is in the range of 0.02 pixels and the relative error accuracy of the radius is in the range of 0.05 pixels. Through the actual measurement of several different parts, the experimental results show that, compared with the manual measurement value, the relative error of the improved algorithm and the original algorithm is lower, the actual value obtained is closer to the manual measurement value, and the measurement accuracy is obviously higher than that of the traditional Zernike moment algorithm. The measurement accuracy can meet the accuracy requirements of the production process of industrial parts.