The invention discloses a manufacturing method of a blended flat affinity filter membrane. The manufacturing method comprises the following process steps of: (1) synthesis of a modified polysulphone substrate membrane material: firstly, preparing a chloromethylated polysulfone material through a Friedel-Crafts reaction by taking polysulphone as a starting material, and then, carrying out a Gabriel reaction to synthesize the modified polysulphone substrate membrane material with an active reaction group, wherein the synthetic process comprises the two steps of synthesis of the chloromethylated polysulfone material and the synthesis of a polysulphone phthalimide material; and (2) preparation of the blended flat affinity filter membrane: dissolving the dry modified polysulphone substrate membrane material and a polysulphone blend at the blend ratio of (2-4):(6-8) in a second organic solvent, adding an additive after the dry modified polysulphone substrate membrane material and the polysulphone blend are uniformly dissolved, scrapping to form a membrane after removing bubbles in vacuum to obtain a finished product membrane, and placing the finished product membrane into 50% of glycerol to be stored in a hygrometric state.