Abstract: The relation between contents of rare earth cerium and impurity lead or bismuth and the activity coefficient of cerium,lead and bismuth in Ag Cu Zn series brazing alloy was calculated and analyzed by means of thermodynamic method.The thermodynamic calculation results show that the "equivalent activity coefficient phenomenon" will emerge among the activity coefficient of solute in a certain range of cerium for the metallic solution in which Ce,Pb or Bi is as solute and Ag,Cu or Zn as solvent respectively.Under this condition,the activity coefficient of solute has nothing to do with its own concentration.The preliminary theory analysis to this new finding has been done in this paper.