A novel model consisting of basic micro-processes has been developed on the basis of the classic diffusion theory.It is first time that the concept of exchange rate has introduced,and the growing process of surfactant-mediated epitaxial thin-film growth has been simulated with Kinetic Monte Carlo (KMC) technique.The results of simulation found that the exchange reaction of RLA model is a combination of the basic micro-processes.The majorities of exchange are not complete position exchange and the exchange rate doesnt always equal one.Both surfactant atoms and adatoms would diffuse from one layer to another.The diffusion happens mostly between single atoms and the diffusing atoms increase with substrate temperature or film thickness increasing.