The detection of the change of concentration of cupric ions (Cu2+) 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 (CuSO4) solutions of different concentrations and then ethylene diamine tetraacetic acid (EDTA) solution. The intensity changes of typical transmission spectrum with Cu2+ 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 Cu2+.