Long period grating (LPG)-based sensing and measurement is based on the change of wavelength caused by external environment. In this paper, the refractive index sensing characteristics of LPG are simulated and a series of conclusions are obtained. The change in environmental refractive index can cause a change in the refractive index of the cladding, thereby affecting the transmission mode in optical fibers, ultimately manifested as changes in the center wavelength and intensity of the grating resonant peak. Measuring the variation of its spectral resonance peak can directly obtain the refractive index of the solvent, and then indirectly obtain chemical parameters such as solvent concentration based on empirical formulas. In this paper, NaCl solution with different concentration are be measured, and then the concentration of the solution can be obtained. Refractive index measurement technology by using LPG has important research significance.