The invention discloses a method for separating high-density polyethylene, polypropylene and polyethylene glycol terephthalate from multi-component mixed plastic by flotation. The method comprises the following steps: step 1, preprocessing; step 2, crushing and sieving; step 3, performing flotation for the first time; and step 4, correspondingly processing a first floating product and a first sinking product, wherein the first floating product is washed and dried, so as to obtain the high-density polyethylene; and the first sinking product is processed by the following steps: step 401, performing flotation for the second time to obtain polypropylene; step 402, performing flotation for the second time; step 403, performing alkaline treatment; and step 404, performing flotation for the fourth time to obtain polyethylene glycol terephthalate. The method is simple in steps; high-density polyethylene, polypropylene and polyethylene glycol terephthalate can be separated from the multi-component mixed plastic; in addition, the recovery rate and the purity of polyethylene, polypropylene and polyethylene glycol terephthalate are high. Therefore, the recycling rate of plastic wastes can be increased, and moreover, the environmental pollution is reduced.