The invention relates to a high-water-flux forward-osmosis composite membrane and a preparation method thereof. The forward-osmosis composite membrane is a polysulfone-sulfonated polysulfone-inorganic filler blended/polyamide composite membrane. A traditional osmosis membrane is generally applied to a reverse-osmosis system, and if the traditional osmosis membrane is used in a forward-osmosis process, the water flux of osmosis is far lower than a theoretical value since the traditional osmosis composite membrane is large in thickness, the hydrophilicity of a supporting layer of a polyamide layer is poor so as to cause serious inner concentration polarization. The preparation method comprises the following steps: blending a polymer with a modifier to form film casting liquid; after scraping or leveling, performing non-solvent coagulating bath with water; by a phase-inversion method, preparing a polysulfone ultrafiltration membrane with good hydrophilicity, high porosity, uniform surface pore structure and narrow pore diameter distribution; and in a clean environment and at room temperature, airing the polysulfone membrane till the surface is dry, and growing a polyamide active layer on the surface of the polysulfone membrane by an interfacial polymerization method, thereby obtaining the forward-osmosis composite membrane with good hydrophilicity, high porosity, low thickness and high mechanical strength. The high-water-flux forward-osmosis composite membrane disclosed by the invention has the advantages that not only the osmosis flux is high, but also the reverse leakage of an extraction medium is low, and the selective permeability is good.