In order to numerically study the reaction characteristics of magnesium and vapor in broad temperature range, the reaction model based on diffusional and chemical dynamic control respectively was established. The diffusional control model considered the partial condensation of gaseous MgO on the surface of magnesium droplet. The chemical dynamic model of Mg/H₂O reaction was based on the Arrhenius equation. The reaction rate of Mg/H₂O at different conditions was studied numerically. The results show that the calculated results are consistent with the experimental results. The reaction rate is great at high temperature and concentration of vapor, but low at high pressure. The reaction rate of magnesium and vapor is controlled by different factors at different temperature region. The reaction rate is controlled by chemical dynamic below the critical transition temperature, but by diffusional dynamic above the critical transition temperature.