Based on the three-layer fiber model and the coupling theory, the effective refractive index of the core mode and the cladding modes were numencally calculated in Long-period grating (LPG)，and the changing relation of the coupling coefficient of the cladding modes at the lowest few ones with varying wavelength were analyzed. It is revealed that the mode number of the maximum coupling coefficient of the cladding modes decreases with wavelength, the coupling coefficient with wavelength is different in different fiber parameters. These results are valuable for the design of long period grating, since the coupling coefficient directly affects the peak loss of the grating transmission spectrum.