A fluorescent fusion protein is provided. The fluorescent fusion protein of the invention comprises a first fluorescent protein with first emission wavelength, a second fluorescent protein with second excitation wavelength and second emission wavelength, and a linker peptide linking the first fusion protein and the second fusion protein, wherein the first emission wavelength overlaps the second excitation wavelength so that the second fluorescent protein exhibits the second emission wavelength after receiving energy. The fluorescent fusion protein of the invention exhibits a stronger fluorescence in the absence of bilivirdin (BV).
