To solve the technical problems of hot metal desulfurization by injecting magnesium particulate, a new method of hot metal desulfurization by bottom-blowing magnesium vapor combined with mechanical agitation was put forward. The effects of three different desulfurization processes on the desulfurization efficiency were studied in view of thermodynamics and kinetics. It was found that the utilization efficiency of magnesium can reach 82.6％ and desulfurization efficiency can reach 86.2％ during the first 4 min using the method of magnesium vapor injection combined with mechanical agitation. The gasification of magnesium powder leads to significant splashing and magnesium losses in the process of magnesium powder injection, resulting in a low utilization efficiency of magnesium of 51.8％ and a low desulfurization efficiency of 55.76％. Activation energy for a first-order kinetic relationship between magnesium powder and sulfur was measured from the experiments, which was 142.82 kJ/mol in the temperature range of 1573-1723 K. The activation energy of the reaction between magnesium vapor and sulfur was around 54.8-65.0 kJ/mol in the temperature range of 1573-1723 K, which indicates that the desulfurization with magnesium vapor proceeds relatively easier than the desulfurization with magnesium powder.