The invention discloses a method for extracting plant protein from cassava starch waste water, wherein starch-containing yellow serofluid waste water discharged in a process for producing starch by utilizing cassava is fully utilized. The method is characterized by comprising the following steps of: 1, introducing the yellow serofluid waste water into a reaction sedimentation tank by virtue of a premixing groove, metering and throwing protein precipitant and lime water into the premixing groove, controlling alkalinity of water in a pond and controlling settling time to be 1-2 hours, so as to produce precipitate; 2, introducing the precipitate into a stirring pond, adding ferrous-iron-containing salt and uniformly stirring to obtain dissolved matter; 3, pumping by virtue of a pump to convey the dissolved matter into a pressure filter to be subjected to filter pressing, so as to obtain filter residue; and 4, baking the filter residue and smashing the filter residue into powder, and packaging to obtain finished products. The method disclosed by the invention has low cost and simple process, reaction of protein to certain medicine is utilized, the plant protein is condensed and separated under the condition of slight alkalinity, the added drug and the protein are condensed into a whole and precipitated, the products are recycled by virtue of a machine, water is discharged and no drug residue is left, pollutant is greatly reduced, and follow-up biochemical treatment can be beneficially carried out.