Abstract: Compared with traditional measurement methods, digital speckle correlation method(DSCM) can better meet measurement requirements of small angle displacement in different occasions due to flexibility of its target feature unit meshing. In order to solve problem of sub-pixel small-angle displacement measurement, nine-point quadratic surface fitting method of sub-pixel measurement image is studied in this paper, and optimal error is obtained by computer simulating experiment. Optimal speckle size of sub-pixel small angle displacement is calculated by surface fitting method under condition of efficiency optimization. Calculation window size is 41×41 pixel and window size is 3×3 pixel. Experimental results verify validity of above parameters, and provide reference for further measurement of angular displacement of digital speckle image by surface fitting method.