A new method for separating principal Strains of photoelastic coatings has been presented in this paper. Drilling small holes in the coatings, in directions of principal strains, taking approximately the ratio of fringes at the hole edge equal to the ratio of principal strains，principal strains can be obtained. In the paper, the principle and approximate equation and typical experiment have been researched. The method is visible and simple. It is shown that the method is easy to handle and practical value in engineering.