A polymer photonic crystal biosensor with a period of 1000 nm is proposed on a SiO2 substrate. Then a high refractive index material is deposited on the surface of the photonic crystal. The rigorous coupled-wave theory is employed to analyze the characteristics of the narrow-band infrared reflection spectrum of the photonic crystal biosensor, and the relation between the peak reflection wavelength and the effective refractive index is established. With concentration of the analyte adsorbed on the surface of the photonic crystal varying, the effective refractive index changes simultaneously, which leads to a shift of the peak reflection wavelength. Therefore, the distribution of the analyte according to the amount of shift of the peak reflection wavelength is obtained. The influence of the coating with different refractive indices, thicknesses and filling ratios on the reflection spectrum and sensitivity of the shift of the peak reflection wavelength is also studied. The results indicate that the sensitivity of the biosensor can be improved by selecting appropriate parameters of the coating.