Energy concentration degree which can evaluate the beam quality is significant while analyzing the energy transmission of the beam. The accurate integral expression of energy concentration degree η of nonparaxial scalar beam diffracted by circle aperture is deduced by accurate definition of light intensity. Detailed numerical calculations are carried out to discuss the law of η versus diffraction aperture ρο and transmission distance z. It’s shown that η increases monotonously with the increase of z only under some conditions. η does not increase monotonously with the increase of ρο and it gets the extremum if ρο is taken as a special value. The approach to get the largest energy and energy concentration degree of a spot is given by controlling the parameters ρο and z．