Beam sampling grating (BSG) is an important optical element applied in the terminal optical system of inertial confinement fusion (ICF) drivers. It can separate a very slight part of laser energy (-0.1%) from the main beam,thus provide sampling signal for the precise diagnosis of the laser parameters. However,in application,there are always certain deviations between the incident beam lighting on the BSG′s face and the ideal one,such as the lean of incident angle,wavefront distortion,etc.. These deviations affect the accuracy and validity of BSG′s sampling signal. In this paper,a model of BSG′s using state is established based on the theory of Fresnel holographic recording and representation,and the process of beam sampling is further studied. This paper emphatically analyzes the location and energy distribution of sampling spot when light wavelengths have wavelength shift,incident angle deviation or wavefront distortion. The analysis results of this paper provide theoretical reference for practical engineering of BSG in application system.