Long-period fiber gratings(LPFG) can be used in the field of communication and sensing,especially measurement of flammable,explosive,EMI environment and caustic,venenous liquid because of their excellent properties such as anti-magnetism,anti-noise,anti-explosion and so on.A metal film was plated on the surface of LPFG cladding.Based on LPFG sensing principle and its intrinsic equation,the modeling and simulation were carried out,and the influences of the parameters such as number of mode,period of LPFG,film thickness on harmonic peak wavelength were discussed.The obtained conclusion can provide a reference for selecting rational structure parameters of LPFG used for sensing measurement of refractive index.