To study the deformation of an optical surface and its effect on optical performance, a method based on the finite element method for calculating the area of a particular deformation region of an optical surface is proposed, and a concept of effective area ratio is further introduced to describe the effect of deformation on optical performance. The principle of the method is presented, and the deformation of a potassium dihydrogen phosphate (KH2PO4, KDP) crystal is studied using this method, while the calculation accuracy is discussed. The results demonstrate that the method and the concept of effective area ratio are available for studying the deformation of an optical surface and its effect on optical performance.