The detection of the change of concentration of cupric ions (Cu<sup>2+</sup>) by long-period grating (LPG) sensors was presented in this paper. A polymeric affinity coating was chemically attached to the surface of the LPG sensor with reaction times of 4 hours, 8 hours, and 12 hours, which is characterized of its surface appearance by scanning electronic microscope (SEM). By comparing the surface appearance of the polymeric affinity coating, a proper reaction time was chosen as the final experimental condition. The optimal experimental condition was applied to the LPG sensor, which was first dipped into cupric sulfate (CuSO<sub>4</sub>) solutions of different concentrations and then ethylene diamine tetraacetic acid (EDTA) solution. The intensity changes of typical transmission spectrum with Cu<sup>2+</sup> concentrations were studied. Results showed that the intensity-based method of LPG sensor can reflect the change of the output light power and can be used to indicate the concentration of Cu<sup>2+</sup>.