Based on the coupling theory and the simplified three-layed model

the influence of the surround refractive index change upon an long period optical grating（LPFG）is studied

its characteristics such as resonant wavelength and peak loss in detail is analyzed.Relation of wavelength shifts in the LPFG and different concentrations of sodium chloride solution is approximate linear

which is agree with the theoretical results.This method is simple

and the signal is wavelength modulated

which is not reflected by the fluctuation of light intensity and the fiber loss.The sensing system is all light transmitted by fiber.It not only can on-line measure

realize rapid accurate measurement for target medium

but also can be used in some special situations and easily realize long distance measurement.