The invention provides a fluorescent biosensor for detecting Hg<2+>. The fluorescent biosensor comprises a composite probe Q, an annular template CP, a trigger chain T, a fuel chain F, a Klenow fragment and thioflavin T; and the trigger chain T, the composite probe Q, the fuel chain F, the annular template CP and the Klenow fragment are uniformly mixed with a mercury ion solution with a series ofconcentrations and a to-be-detected solution respectively, incubating is performed, enzyme is inactivated, the sulfur element T is added, incubating is performed, and the fluorescence intensity is detected. Target cyclic amplification and signal amplification are realized on the basis of the specific binding capacity between basic groups T and Hg<2+> and entropy-driven strand displacement amplification and rolling circle amplification reactions, and the biosensor for sensitive detection of a target object is realized. The sensor has the advantages of high detection speed, low detection limit,high sensitivity and the like, can make up the defects and deficiencies of the existing detection method of Hg<2+>, and realizes rapid and accurate quantitative detection of Hg<2+>.