In this paper,by means of the Collins formula and boundary finite element method,diffraction integral equation of the square resonator with spherical mirrors has been transformed to the finite order matrix equation,and mode fields of the square plano concave resonator with a Gaussian reflectivity mirror have been calculated.As a result of numerical calculation,the square plano concave resonator with a Gaussian reflectivity mirror has good choosing capability for transverse modes,which can be applied in high power lasers to acquire good quality of laser beams.