The invention discloses a preparation method of a flat membrane for sludge concentration, and belongs to the fields of sludge concentration flat membranes. The method comprises the following steps: sequentially adding a solvent, a pore-foaming agent, a surface active agent and polyvinylidene fluoride (PVDF) into a wild-mouth bottle according to certain ratio, and meanwhile performing stirring, soas to uniformly mix the materials, then putting the wild-mouth bottle into a 50 to 80 DEG drying oven, and timely stirring every day, so as to obtain a uniformly mixed casting solution, uniformly coating the casting solution onto a non-woven fabric base material through a flat film blade, soaking non-woven fabric into a coagulating bath, so as to perform phase transformation, soaking a flat membrane into glycerinum, then treating the flat membrane, and performing normal temperature drying treatment, so as to obtain a PVDF membrane used for sludge concentration treatment. The PVDF membrane hasthe characteristics of high flux, high strength and low pollution rate, can ensure that water yielding is stable when being applied to a flat membrane sludge concentration reactor, and properly meetsthe requirement on sludge concentration.