The invention discloses a compound desulfurizer and a single-injection particle magnesium molten iron compound desulfurization method, and belongs to the technical field of steel and iron metallurgy. The compound desulfurizer comprises the following components in percentage by mass: 28-58% of lime, 1-3% of fluorite, 38-68% of particle magnesium, and 0.3-1% of graphite powder. During desulfurizing, the compound desulfurizer is firstly fed in a storage tank; then, the compound desulfurizer in the storage tank is loaded to an injection tank; and finally, the compound desulfurizer in the injection tank is injected into molten iron for desulfurization. The compound desulfurizer and the single-injection particle magnesium molten iron compound desulfurization method can improve the viscosity of molten iron slag to enable the slag to locate in a loose state, facilitate slagging, are excellent in desulfurization effect and low in consumption of the desulfurizer, reduce the cost consumption, need no equipment upgrading, and in particular, are suitable for desulfurization process upgrading of enterprises adopting the single-injection particle magnesium desulfurization.