Based on the rigorous coupled wave theory, the narrowband reflective characteristics of photonic crystal are analyzed. The mathematical model about reflection peak wavelength and media concentration adhered on the photonic crystal surface is established. The reflection peak wavelengths (PWV) are different, when the concentrations of the adhered molecules are different, so we can know the concentrations of the adhered molecules by reflection wavelength through using Michelson interferometer imaging reflection space spectrum of photonic crystal. From the measurements of several typical biochemicals molecular concentration improved that photonic crystals narrowband reflection wavelength and molecular concentrations are in a linear relationship, and the reflection wavelength has a red-shift with the increase of molecular concentration. When the molecules are different, the sensitivity of the biosensor is different as well.