Utilizing the shape characteristics of object in multi-slits autocollimator and imaging characteristics of photo-electronic detector,the pixel edge of image filtered by gauss filter is detected by the algorithm which can be used to detect the edge of image.Based on it,local Newton interpolation functions are used to detect the sub-pixel edge of image.Furthermore,least square beeline edge fitting is used to improve detecting precision of the sub-pixel edges of image.After testing,0.13 arc second precision is achieved by the algorithm.