Abstract: Beam sampling grating is an important diffraction optical element for beam sampling diagnosis. Using two gratings as initial optical structure instead of single grating, an aberration-free structure was designed by using Zemax optical design software, which is a faster and more flexible tool for designing aberration-free grating-pair than Matlab. The method can verify the calculation result of the program. Design results show that the RMS radius of the converging light diffracted by two gratings is only 0.506m in spot pattern image, compared to 7.284m for the single grating, which indicates that two gratings can reduce aberration and improve image quality. Besides, the optical system can be used to detect far field beam, which provides effective technical support for online diagnosis for laser beam characteristics.