The invention discloses a molten iron pretreatment compound dephosphorization agent with converter slag as the raw material, and the compound dephosphorization agent contains the following raw materials by weight: 45 to 55 percent of converter slag, 3 to 5 percent of quartz sand, 22 to 34 percent of steel-rolling iron sheet, 3 to 6 percent of bauxite, 10 to 14 percent of soda and 0 to 5 percent of fluorite; the raw materials meeting the ingredient demands are evenly mixed according to the proportions after being crushed, and then the product with the diameter being 20 to 50mm is produced after ball-milling, screen separation, palletizing and drying. The compound dephosphorization agent realizes cyclic utilization of the converter slag which accounts for 45 to 55 percent, thus reducing the produced slag amount in the steel-making process and the environmental load; the converter slag contains plenty of CaO and FeO, which can reduce consumption of lime to a certain degree, thus increasing the metal yield rate and reducing the steel-making cost; the compound dephosphorization agent has low melting point, slag can be formed fast, the liquidity is good, thus shortening the treatment time, the dephosphorizing efficiency is high, and the dephosphorized final slag has low alkalinity and low melting point.