we propose a method to estimate the diffusion spot energy of the detection system. we do outdoor observation experiments in xinglong observatory, by using a detection system which diffusion spot energy concentration is estimated (the correlation coefficient is approximate 0.9926). the aperture of system is 300mm and limiting magnitude of system is 14.15mv. obervation experiments show that the highest detecting magnitude of estimated system is 13.96mv, and the average detecting magnitude of estimated system is about 13.5mv. the results indicate that this method can be used to evaluate the energy diffusion spot concentration level of detection system efficiently.