The invention provides an ultrafiltration membrane and a preparation method thereof. The preparation method comprises the following steps: (A) dissolving a polymer and an additive in an organic solvent to obtain a casting membrane solution, wherein the polymer is prepared from one or more of polyacrylonitrile, polysulfone and polyvinylidene fluoride; (B) adjusting the viscosity range of the casting membrane solution to 800 to 30000mPa.S; (C) carrying out membrane scraping on the casting membrane solution and solidifying in a coagulation bath of a composite solution to obtain a wet membrane, wherein the composite solution is prepared from the organic solvent and water; (D) drying the wet membrane in drying liquid to obtain the ultrafiltration membrane. According to the preparation method disclosed by the invention, the viscosity is adopted as a control index to adjust and control relevant preparation conditions in the preparation process of the ultrafiltration membrane; the method is simple and practical. Meanwhile, the composite solution containing the organic solvent and the water is used for solidification, so that the formed membrane pore structure is uniform, the membrane fluxis increased and the separation efficiency is improved.