This paper puts forward the finite element method for solving the Fredholm-Ⅱ equation with continuous kernels. The numerical results of loworder modes obtained by applying this method to the practical problems of optical resonators with infinite strip plane mirrors, inclined strip plane mirrors and circular mirrors are same as that obtained by using the Fox and Li's method.Moreover,by applying this method, we have gbtained some unpublished field distributions and diffraction loss curves of many high-order modes.