NOVELTY - Cyclically liquefying a mixed supercritical fluid of a plant biomass and a waste polyester comprises liquefying by placing plant biomass, polyester, catalyst, and a monohydric alcohol liquefying agent in a kettle, warming to 240-280 degrees C, and maintaining the temperature for 5-40 minutes, naturally cooling to 100 degrees C, cooling to room temperature, and filtering; adding liquefying agent to liquefied liquid; mixing plant biomass, polyester, and catalyst, warming to 240-280 degrees C, cyclically liquefying, and filtering; and extracting with an organic solvent, leaching slag, and distilling at 35-40 degrees C.    USE - Method for cyclically liquefying mixed supercritical fluid of plant biomass and waste polyester to prepare biological oil (claimed).    ADVANTAGE - The method has an improved liquefying rate of the plant biomass and utilization rate of the waste polyester by cyclical liquefication, reduces dosage of the liquefying agent, reduces carbonizing and wall-sticking condition, reduces molecular weight of the liquefied product, and improves quality of the biological oil.    DETAILED DESCRIPTION - Cyclically liquefying a mixed supercritical fluid of a plant biomass and a waste polyester to prepare biological oil, comprises:    (A) liquefying by placing plant biomass, polyester, 1-10 mass % catalyst, and 4-8 times liquefying agent in a reaction kettle with a stirrer, where the liquefying agent is 1-4C monohydric alcohol, the mass ratio of the plant biomass to polyester is 2/1-2/4, warming the kettle to 240-280 degrees C and 5-8 MPa, and maintaining the temperature for 5-40 minutes, where cyclical liquefying time is 1-3 times;    (B) naturally cooling to 100 degrees C, adding cooling water to accelerate cooling speed, cooling to room temperature, and filtering to obtain liquefied liquid of the biomass and residual slag;    (C) adding liquefied liquid to the kettle, and adding the liquefying agent;    (D) adding the plant biomass and polyester to the kettle, adding the catalyst, uniformly stirring, warming to 240-280 degrees C and 5-8 MPa, maintaining the temperature for 5-40 minutes, carrying out cyclically liquefying, naturally cooling to 100 degrees C, adding cooling water, cooling to room temperature, and filtering;    (E) taking the liquefied liquid as liquefying agent, repeating operations for 1-3 times to obtain cyclical liquefied liquid of the biomass; and    (F) extracting the cyclical liquefied liquid of the biomass with an organic solvent, leaching the residual slag, and distilling the extracting liquid and leaching liquid after combined at 35-40 degrees C.