The matching radius of fiber probes with given conditions ation based on the theory of mode matching.The ray tracing of exciting laser propagation in the tapered probe with the calculated matching radius is analysed.The analyzing results indicate: if the probe coupling beams were without limitation,some rays would not satisfy the conditions of total internal reflection when they were propagated in tapered probe and would leak out the probe. In this case,the biosensor system would get false detection result and affected the system reliability. It presented this problem and resolved it in the system designing procedures.