A novel ambient refractive index sensor based on long period waveguide grating(LPWG)with a higher refractive index overlayer is proposed and investigated. The response of LPWG sensor to ambient refractive index change is simulated by studying the shift of resonance wavelength caused by the corresponding change of effective refractive index of a certain higher-order mode. It is shown that the deposition of the overlayer will lead to the re-organization of high-order modes and then an abrupt change of the effective refractive index of the higher-order mode,which will greatly improve the sensitivity and operation range of LPWG refractive index sensor.