Backscattering spectrometry is used to systematically investigate the temperature, dose and dose-rate dependences of Mo/Si atomic mixing and reaction induced by 170 keV Ar+ ions and 300 keV Xe2+ ions. These results show that the ion beam mixing mechanism of Mo/Si system is not vacancy diffusion and intra-cascade interstitial diffusion, and the thermal spike model also does not apply. We suggest that thhe mechanism of Mo/Si atomic mixing induced by ion implantation should be interstitial diffusion and give a reaction model. Based on this concept, the experimental results can be explained.