A Long Period Fiber Grating(LPFG) whose period is 400 μm was fabricated by amplitude mask technique. The end of the fiber, which is 6 cm from the LPFG, was coated with Ag. Taking NaClO3 aqueous solution for example with same amount of NaClO3 powder added into the solution by every time, a new device and method for immediately calibrating saturation point under a specified ambient temperature was studied. Based on the principle of phase difference and solving the eigen equation of core mode and cladding mode of single-mode fiber, the reflectance spectrum variation of LPFG-Michelson interferometer induced by ambient refractive index change was analyzed. By reference to variation tendancy of wavelength shift value with refractive index change, the change curve of the notch wavelength shift induced was obtained by repeatedly add NaClO3 powder into the solution. The experiment value is fitted by least square method, and the theoretical saturation point of NaClO3 aqueous solution under 32℃ is obtain by solving intersection point of the fitted curves, which the difference between the result and literature is only 1.5%.