A novel type of fiber optic biosensor（FOBS）is designed based on the theory that the loss of the light intensity in the optic fiber is related to the effective refractive index of the environment.The biosensor has the advantages of simple structure,easy operation,low cost and high sensitivity.The structure of the biosensor is analyzed theoretically in this paper.The sensing properties of the biosensor are studied by detecting various concentration of glucose solution.The results show that the sensitivity of the biosensor is micro mol per litre and the biosensor has good linearity.