The pressure-induced long period fiber grating(LPFG) is formed by pressing the composite waveguide,which consists of a single-mode fiber with a portion of its coating stripped away,and the medium to be measured within a teflon-cannula from double sides with two identical periodically grooved plates.The influence of medium with different refractive indices on the transmission spectrum of the LPFG is investigated experimentally.The results show that the spectrum of the composite waveguide-based grating presents dual-peak resonance,and the central resonant wavelength shifts towards short wavelength as refractive index increases.The higher order of cladding mode is,the greater of the central resonant wavelength movement of the LPFG will be.The central resonant wavelength of the LP14 cladding mode exhibits a total blue shift of 7.2 nm,which corresponds to a sensitivity of 2.78×10-4 RIU,when refractive index changes from 1.33 to 1.434.