The invention relates to the technical field of plastics and discloses a synthetic method of polyimide foamed plastics. The synthetic method is characterized in that PPMA (Polymethyl Methacrylate), polylactic acid, polylactone, polystyrene and the like are reasonably fused into a molecular main chain of polyimide, and a fused matter is gradually decomposed to form polyimide foams at a high temperature state.