Based on the coupling-mode theory, the changing relations of the effective refractive index and its AC/ DC coupling coefficient in different order of cladding modes in long period gratings, versus the varying of wavelength and the order, are numerically calculated and analyzed. It is revealed that the effective refractive index and the DC coupling coefficient, as well as the AC coupling coefficient of odd order modes, will be decreased with the increase in wavelengths. It is also shown that changing rates of the effective refractive index and the ac coupling coefficient of odd order modes corresponding to higher orders are bigger than that corresponding to lower orders. The effects of DC coupling coefficient on the resonant wavelength peak are further analyzed, and the reason that the difference between adjacent resonant wavelength peaks is gradually getting bigger with wavelength increase is finally revealed.