A method for zero-discharge phosphatization and saponification based on a high-pressure closed circulation system, comprising an autoclave, a separation kettle, a buffer kettle, a hydraulic pump and a recovery vat; the method comprising the following steps: step 1: a workpiece is fed into the autoclave and then sealed such that the pressure range of the autoclave is above 20 MPa; step 2: degreasing and derusting by CO 2: carbon dioxide is buffered by the buffer kettle and then delivered to the autoclave by the hydraulic pump, and then the workpiece is derusted and degreased by spraying a high-pressure carbon dioxide gas stream via a Venturi nozzle in the autoclave; step 3: cyclic separation: supercritical CO 2 is delivered to the autoclave for cyclic cleaning by means of the buffer kettle and the hydraulic pump until the autoclave is clean, and oil stains and solid residues are pushed into the separation kettle by the supercritical CO 2; step 4: high-pressure phosphatization: a phosphatizing solution is pressurized and injected into the autoclave by the hydraulic pump, and then mixed and diluted with high-pressure fluid in the autoclave under the action of a stirrer of the autoclave; the autoclave is warmed up to the reaction temperature to phosphatize the surface of the workpiece; the phosphatizing solution undergoes a chemical reaction on the surface of the workpiece to form a phosphatized film; and after the reaction is completed, the fluid in the autoclave is discharged to the separation kettle to separate and collect residues obtained after phosphatization; step 5: cyclic separation II: supercritical CO 2 is delivered to the autoclave for cyclic cleaning by means of the buffer kettle and the hydraulic pump until the autoclave is clean; step 6: high-pressure saponification: a saponifying solution is pressurized and injected into the autoclave by the hydraulic pump, and then mixed and diluted with high-pressure fluid in the autoclave under the action of the stirrer of the autoclave; the autoclave is warmed up to the reaction temperature to saponify the surface of the workpiece; the saponifying solution undergoes a chemical reaction on the surface of the workpiece to form a saponified film; and after the reaction is completed, the fluid in the autoclave is discharged to the separation kettle to separate and collect residues obtained after saponification; step 7: cyclic separation III: supercritical CO 2 is delivered to the autoclave for cyclic cleaning by means of the buffer kettle and the hydraulic pump until the autoclave is clean; and step 8: drying: the autoclave is opened for aeration drying. step 1: a workpiece is fed into the autoclave and then sealed such that the pressure range of the autoclave is above 20 MPa; step 2: degreasing and derusting by CO 2: carbon dioxide is buffered by the buffer kettle and then delivered to the autoclave by the hydraulic pump, and then the workpiece is derusted and degreased by spraying a high-pressure carbon dioxide gas stream via a Venturi nozzle in the autoclave; step 3: cyclic separation: supercritical CO 2 is delivered to the autoclave for cyclic cleaning by means of the buffer kettle and the hydraulic pump until the autoclave is clean, and oil stains and solid residues are pushed into the separation kettle by the supercritical CO 2; step 4: high-pressure phosphatization: a phosphatizing solution is pressurized and injected into the autoclave by the hydraulic pump, and then mixed and diluted with high-pressure fluid in the autoclave under the action of a stirrer of the autoclave; the autoclave is warmed up to the reaction temperature to phosphatize the surface of the workpiece; the phosphatizing solution undergoes a chemical reaction on the surface of the workpiece to form a phosphatized film; and after the reaction is completed, the fluid in the autoclave is discharged to the separation kettle to separate and collect residues obtained after phosphatization; step 5: cyclic separation II: supercritical CO 2 is delivered to the autoclave for cyclic cleaning by means of the buffer kettle and the hydraulic pump until the autoclave is clean; step 6: high-pressure saponification: a saponifying solution is pressurized and injected into the autoclave by the hydraulic pump, and then mixed and diluted with high-pressure fluid in the autoclave under the action of the stirrer of the autoclave; the autoclave is warmed up to the reaction temperature to saponify the surface of the workpiece; the saponifying solution undergoes a chemical reaction on the surface of the workpiece to form a saponified film; and after the reaction is completed, the fluid in the autoclave is discharged to the separation kettle to separate and collect residues obtained after saponification; step 7: cyclic separation III: supercritical CO 2 is delivered to the autoclave for cyclic cleaning by means of the buffer kettle and the hydraulic pump until the autoclave is clean; and step 8: drying: the autoclave is opened for aeration drying.