The invention relates to a blast furnace external molten iron pretreatment desiliconization agent, and a preparation method and a use method thereof. The desiliconization agent is a composite agglomerate composed of an inner mixture layer and an outer mixture layer, wherein the inner mixture layer comprises, by mass, 0-40% of blast furnace fly ash, 0-25% of coking fly ash, 10-30% of fluorite powder, 0-5% of a binder, and the balance of one or more of sintered fly ash, converter fly ash and steel rolling iron sheet; and the outer mixture layer comprises, by mass, 50-85% of iron concentrate, 10-45% of quicklime and 0.5-10% of the binder. The problems of sever temperature decrease of molten iron, caused by low desiliconization efficiency and long time in the prior art, are solved, the self solid waste resources of steel enterprises are fully utilized, a special device or a blowing device does not need to be additionally mounted, and extra purchasing of a special desiliconization agent material is not needed, so cheap, simple and effective desiliconization of molten iron is achieved.