Nickel ions (Ni²⁺) probe with enhanced sensitivity is experimentally d emonstrated by using a normal fiber Bragg grating (FBG) cascaded with coating-functionalized fiber tip.The coating of chi tosan/poly acrylic acid (PAA) is formed directly onto the cleaved fiber tip via layer by layer assembly.When the Ni²⁺ is absorbed by the chitosan/PAA coating,the Fresnel reflectivity of the fibe r tip,which forms the noise background in the FBG′s reflection spectrum,can be modulated due to Ni²⁺-caused refractive index variation of the coating.T herefore,by tracing the variation of the signal-to-nois e ratio (SNR),measurement of Ni²⁺ concentration is achieved successfully with maximum sensitivity of 35.52dB/mM.By using the SNR demodulation method, we can not only eliminate the influence of power fluctuation of the light source on the demodulation accuracy and stability, but also allow independent temperature measurement through Bragg wavelength shif t of the FBG.The heavy metal ions sensor based on SNR measurement of fiber Bragg grating h as the advantages of low cost,miniature size,light weight and immunity to electromagnetic,which demonstrates a promising potential in the field of measurement of heavy metal ions.