Hot-dip aluminizing panels of Q235 steels were produced in laboratory. Then the hot-dip aluminizing samples were executed diffusion at elevated temperatures protecting with flowing argon gas. Metallographic microscope, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) were employed for compositional analyses and graphical analyses of coating. The diffusion process was researched and the phase’s microstructure after diffusion was analyzed and discussed. The hot-dip aluminizing diffusion mathematical model was established with the finite element method. The results show that after diffusion in hot-dip aluminizing, the aluminum of the coating disappeared. The thickness of diffusion layer increased and the dentate frame disappeared. There are some iron-rich phases such as Fe2Al5, FeAl and Fe3Al in the diffusion layer. The phase Fe2Al5 decreases and the phase FeAl increases. The mathematical model corresponds with the actual situation. The diffusion coefficients of Al atoms diffusing in intermetallic compounds were calculated at 950°C, which were 0.29×10-12 m2/s for Fe2Al5, 0.7×10-12 m2/s for FeAl, and 0.27×10-12 m2/s for Fe3Al, respectively.