The wavelength variation of Fiber Bragg Grating (FBG) is affected by the effective refraction ratio and fiber Bragg grating period . From measuring the change of reflected wavelength of FBG, we can ascertain the change of environment. We suppose that with pressure field and the axial stress field being kept constant, then a temperature sensing model is set up, and in combination with demand of measuring the water temperature in front of the Three Gorges Dam , we suggest using the quadratic relationship of temperature and wavelength. The calibration results can meet the requirements, and the monitoring results are good.