Aiming at restricting the number and the thickness of elements used in the inertial confinement fusion (ICF) system and utilize the characteristic of easy integration of diffractive optical element (DOE)，we can integrate different DOEs in one optical element for the system optimization in the final optics assembly. In order to lay a theoretical foundation for normal running of combined elements used in the ICF system under strong laser irradiation， this paper uses Fourier modal method to simulate the modulation characteristics of single color separation gratings (CSG) and the color separation gratings-beam sampling gratings (CSG-BSG) combined optical elements in the near field， respectively. We found that the maximal modulation of every layer of CSG-BSG combined optical element was 10% to 47% smaller than that of single CSG. However， the probability of laser induced risk for CSG-BSG combined optical element is also in the interface of every layer as in single CSG. In addition，through comparison of the simulated distribution of optical amplitude in the area from exit surface to 1μm for CSG and CSG-BSG elements with the cross section distribution of optical amplitude after transmitting 0.001 and 0.800μm for the two optical elements， it was shown that CSG-BSG combined optical element can realize both harmonic separation and beam sampling， but hardly change the exit optical field distribution. So CSG-BSG combined optical element can meet the requirement for the ICF driver.