The invention relates to a preparation method of a liquid carbon dioxide thickening agent for unconventional oil and gas reservoir development, and can solve the problem of damage to an oil and gas reservoir in a conventional mining mode. The preparation method has the technical scheme that methylbenzene and water are added into a bottle; then, serinol is added; terbutyloxycarbonyl anhydride is dropwise added at 0 DEG C; reaction is performed for 4h at 25 DEG C; distillation is performed to obtain an intermediate product (1); then, perfluoro caprylic acid is added into another bottle; thionyl chloride and N,N-dimethyl formamide are added at 75 DEG C; reaction is performed for 2h; distillation is performed to obtain an intermediate product (2); next, the intermediate product (1) is added into another bottle; the temperature is raised to 75 DEG C; the intermediate product (2) is dropwise added; reaction is performed for 12h at 75 DEG C to obtain an intermediate product (3); finally, dichloromethane and trifluoroacetic acid are added into the bottle; then, the intermediate product (3) is added; reaction is performed for 2h at 25 DEG C; after extraction and vacuum drying, hexamethylene diisocyanate is added to perform reaction for 2 minutes; a final product is obtained. The thickening agent can increase the carbon dioxide viscosity, and can be used for fracturing reformation.