Abstract: In order to study the energy focus ability of the beam diffracted by a small circular aperture,based on the rigorous vectorial diffraction theory and the accurate expression of light intensity,the power density expression of nonparaxial diffraction beam at the transverse plane,detailed numerical calculation was performed and the different results were compared.It showed that the energy focusability of the diffraction beam depended on the transmission distance,diffraction aperture and facula radius,and appropriate energy focusability could be obtained by controlling some parameters of the optical system.The results have some reference values to the research of energy transmission of the fiber probe.