The invention belongs to the steel metallurgic technical field and in particular relates to a hot metal pretreatment magnesium desulfurization method and the adopted powder, wherein, the powder comprises magnesium metal powder and is characterized in that the powder also comprises premelting magnesium desulfurization accelerating agent which is made from blast furnace slag and lime through production processes such as raw material selection, impurity removal and drying, mix crushing and passivating treatment; by means of the prior blowing equipment, the magnesium metal powder and the premelting magnesium desulfurization accelerating agent are mixed according to a certain proportion or are jointly injected into hot metal for desulfurization. The method is simple and feasible and the made powder can replace pure lime powder, therefore, the invention has the advantages of energy saving, high efficiency and environmental protection; moreover, the method can greatly reduce the pretreatment cost of hot metal and increases hot metal yield.