Shape from shading(SFS)is an important domain in computer vision. The paper presented an improved algorithm of shape from shading based on a single image according to an existed one. The presented algorithm enhanced the boundary constraints to eliminate the rotation distortion on the border of reconstructed object and introduced the factor of brightness error to weaken the influence irradiance equations nonlinearity on reconstructed errors. The reconstructed results verify the performance improvement in terms of accuracy by the input image of a synthetic image and a real image of weld.