In order to positioning the Mark points in PCB to ensure the cutting position of PCB quickly and accurately, an edge extraction algorithm was proposed based on the improved RANSAC. An estimation model was obtained by analyzing the inherent property of the objective model and filtering sample points directionally. The residual analysis was made by extracting sample points with the stratified sampling method. The best model was picked up after times iterations. The efficiency and precision among the leastsquares fitting with weight, Zernike Moments fitting and Improved RANSAC fitting were contrasted and analyzed. The experiments prove that the last method has the better robustness and stability.