In the research work, on the basis of the classic diffusion theory we have advanced a novel model of the reaction limited aggregation (RLA), which involues the basic micro-processes. We firstly introduced the concept of exchange rate and have also simulated the growing process of surfactant-mediated epitaxial thin film growth with the kinetic Monte Carlo (KMC) method. By simulating, it was found that both surfactant atoms and adatoms would diffuse from one layer to another. This kind of diffusion mainly happens beween single atoms and the number of this diffusion increases with increasing growth temperature or increasing deposition depth. The exchange reaction of RLA model is the combination of several basic micro-processes. The majority of exchanges are not complete position exchange and the exchange ratio is not always equal to unity.