A fiber-optic evanescent wave biosensor has been developed, using fiber bundle to transmit excitation laser beam and fluorescence signal. Based on the theory of evanescent wave and mode matching in probe, the geometric configuration of probe is designed, and the detection ability of this sensing system in direct binding is tested... The system is tested in two cases, in one case the probe is put in airs and in the other case the probe is put in water, the signal data is collected to computer by DAQ devices at the same time, then make correlation detection with this data. The experiment results show that the correlation detection method can distinguish the system noise and the detected fluorescence signal clearly. The system has good feasibility.