Abstract: The interaction between rare earth samarium ion and bovine serum albumin(BSA) was investigated by fluorescence spectrum. Effects of the concentrations of rare earth samarium ion and the pH of buffer solutionon the fluorescence intensity were discussed. Fluorescence emission spectrum of bovine serum albumin was scanned with the changing of samarium ion concentration. The results showed that it was a static quenching. Using Stern-Volmer equation and double-reciprocal equation, the quenching constant and binding constant were calculated as follows:K_q=4.12×10¹² L·mol^-1·s^-1, K_a=9.652×10⁵ L·mol^-1, n=1.22.