Aiming at the problems existing in the enzymes, proteins and other biological molecules detection, a kind of defective laddertype optical biosensor has been designed in this paper. The reflection efficiency of reflectance spectroscopy of biological is calculated by using the rigorous coupled wave analysis (RCWA). The simulation result showed that there exists a good linear relationship between the refractive index of the biomolecules and the incident angle of laser at specific peak of reflection spectrum of the biosensor. And there has a good linear relationship between the reflection efficiency of reflection spectrum of the biosensor and the refractive index of the biomolecules at a fixed laser incident angle of 75.4°. The conclusion indicates that not only the reflection efficiency of reflection spectrum of the biosensor but also the incident angle of laser can detect the refractive index of the biomolecules, and then can get the information of biomolecules.