A subpixel edge detection algorithm based on Zernike moment is proposed to detect the size of the valve steam seal. Firstly, the preprocessing effect of the Gaussian filter and bilateral filter is compared and analyzed. The results show that bilateral filter perform better in filtering and edge information retaining. Then, the Canny operator is employed for rough edge localization, and the adaptive Zernike moment transition model is adopted for subpixel level localization. Finally, the coordinates and radius of the center of the circle are obtained by the least squares fitting circle method. Experiments prove that the optimized algorithm has a smaller average error in edge positioning with more accurate accuracy.