Aluminum/magnesium (Al/Mg) alloys have good metallurgical compatibility as well as similar melting and recrystallizationtemperatures. Therefore, the formation of deleterious IMCs is considered as a major problem in dissimilar Al/Mgalloys joining. In this paper, the metallurgical reaction behavior and atomic diffusion in Al/Mg solid state joining interfaceare studied. Based on the thermodynamic, kinetic and crystallographic characteristics of ordered phase transformation andatomic diffusion database, a phase field calculation framework of the polycrystalline β–Al3Mg2 and γ–Al12Mg17 evolutionis developed. Diffusivity, D and interface mobility, M, which significant kinetic parameters are defined as alloy compositionsand joining conditions dependent. By a series of simulation, it is proved the simulated interdiffusion microstructuralcharacteristics and element distribution across the joining interface are in coincidence with the corresponding experimentaldata. The thermally activated IMC evolution in diffusion bonding is in thermodynamic equilibrium, while accelerated IMCevolution in friction stir welding results from thermomechanically activated D and M. The present study contributes to theunderstanding of interfacial microstructures evolution in solid state joining of dissimilar alloys.