Based on the model of triple-clad long-period fiber grating and dispersion equation,the influence of overlay thickness on the resonant wavelength of LPG is studied.It comes to the conclusion that,when the overlay thickness reaches a certain value,the modes with peak effective index in the cladding modes（those with lowest cladding modes）will gradually disappear.The disappearance of these modes will result in the moving forward of subsequent modes,which will cause a leap of the effective index.In correspondence with the changes of the effective index,the shift characteristics of resonant wavelength of cladding modes may fall into three areas with the increase of the overlay thickness.In the first and third areas,the change of the overlay thickness has few effects on the change of the resonant wavelength,but in the second area,the resonant wavelength is quite sensitive to the overlay thickness-a small change can cause large shift of resonant wavelength.An optimum overlay thickness（OOT）exists in this area,and once the overlay thickness equals OOT,the resonant wavelength of LPG would be remarkably subtle to the changes of overlay thickness.