The optical measurement principle of ophthalmic lenses’ vertex power was analyzed.The quantitative formulas for spherical power,cylinder power and cylinder axial angle of the lens under test were derived based on the theory of prism effect.The formulas show that in the condition of fixed optical center,the above three optical parameters of the lens can be measured from detecting the coordinate values of two light spots of holes on the observation screen.The reliability of the formulas was further proved by simulations of one positive and one negative astigmatic power lenses.