The invention discloses a fluorescence sensor and a preparation method and application thereof. The fluorescence sensor is made from a nanocomposite consisting of aminated carbon quantum dots (N-CQDs)and p-aminothiophenol modified silver nanoparticles (N-AgNPs). During preparation, the aminated carbon quantum dots are prepared firstly, then the p-aminothiophenol modified silver nanoparticles areprepared, and finally the N-CQDs/N-AgNPs nanocomposite fluorescence sensor is prepared from the aminated carbon quantum dots and the p-aminothiophenol modified silver nanoparticles. The fluorescence sensor can be used for detecting the content of dopamine (DA) in serum; the prepared fluorescence sensor can detect the dopamine content in a serum sample conveniently and simply; and the method has the advantages of being cheap, simple, high in sensitivity and selectivity, wide in linear range, short in detection time and the like and has huge application prospects in DA detection in the biological analysis or disease research process.