The interaction of L-homocysteine (Cys) modified gold nanoparticles with bovine serum albumin (BSA) was studied by fluorescence quenching spectroscopy and synchronous fluorescence spectra. The binding constant and binding sites of L-homocysteine modified gold nanoparticles to BSA were calculated, respectively, according to the double logarithm regression curve. Fluorescence quenching of BSA by L-homocysteine modified gold nanoparticles was observed, indicating that the quenching mechanism is static quenching. In addition, the thermodynamic data show that the key interaction force is hydrophobic interaction. Finally, the synchronous fluorescence spectra show that the conformation of BSA and the microenvironment of the tryptophan have not obviously changed.