According to the Maxwell equations, the diffraction characteristics of the color-separating grating (CSG) usea in ICF system have been calculated. The coupler-wave analysis and enhanced transmittance matrix have been employed during the calculation process. The results that get by the method, which used in this рapеr, are better than the results that get by the traditional scale method The calculation shows that the shorter of the period of the CSG, the lower of the diffraction efficiency of 3ω at zero order.