Fiber Bragg grating (FBG) cannot be directly used as gas or liquids sensors, because it is not sensitive to external refractive index.By side-polished,the cladding layer of one side of FBG can be reduced to several micrometers.The radiation field can reach the external medium.When the external refractive index changes, the effective refractive index of fiber changes and the center wavelength of D-shaped FBG shifts accordingly.In the experiment,the NaCl solution with different concentration is used to change the external refractive index of FBG.The center wavelength of D-shaped FBG shifts when the concentration of NaCl solution changes.The relationship between the offset of wavelength and the concentration of NaCl solution is approximately linear.In addition,the relationship between the transmitting power and the concentration of NaCl solution is also approximately linear.