The sub-wavelength grating was designed,which adapting the harsh requirement of harmonic waves separating in inertial confinement fusion (ICF) drivers. Using the commercial software Gsolver which was developed on the basis of vector diffraction theory,the parameters of sub-wavelength grating which would influence separate harmonic waves,were analysed. The design method of sub-wavelength grating for separating harmonic waves was studied. These results offer the supports for the sub-wavelength grating design.