Tb(Ⅲ) as a fluorescent probe was used to study the surroundings of metal ions in FP (fibrinolytic principle).There is a high affinity site of Ca^2＋ in every FP molecule and Ca^2＋ ions are located near the Trp.When Tb (Ⅲ)-FP was excited by 222nm and 233nm, it is possible that energy transfer occurs are as follows: Trp→Tb^3＋,Trp→Tb^3＋→Phe,Tb^3＋→Phe,Tb^3＋→Trp.These results indicate that Phe may combine with Tb (Ⅲ).The results of calculations indicate that the distance between Trp and Tb (Ⅲ) is atout 5.56Å.