The invention discloses a thermoplastic plant fiber/carbon dioxide-based plastic blending material and a preparation method thereof. The material comprises the following components in parts by weight: 1-99 parts of thermoplastic plant fiber, 1-99 parts of carbon dioxide-based plastics, 0-10 parts of cross-linking agent and 0-10 parts of plasticizer. The preparation method comprises the following steps: stirring the raw materials in a high-speed mixer for 5-20 minutes; drying the materials at the temperature of 30-60 DEG C for 8-12 hours; controlling the temperature to 100-140 DEG C, and then melt-mixing the mixture in a double-screw extruder for 5-10 minutes; and finally granulating at the temperature of 110-150 DEG C and under the pressure of 0.1-1MPa to obtain the product. The thermoplastic plant fiber/carbon dioxide-based plastic blending material has good processability, excellent thermoplastic processing characteristics and excellent mechanical properties during the process of extrusion molding, blow molding, injection molding and foaming; and the obtained finished product can be completely degradable after being used and is environment-friendly, and can be widely applied to the production and processing fields of chemical industry, mulch, electromechanical industry, building materials, package, environmental protection and the like.