The present invention discloses a method for preparation of lignin-modified whole bio-based PBAT biodegradable plastic, and the method comprises the following steps: (1) whole bio-based PBAT is synthesized; (2) the whole bio-based PBAT and lignin are dried in vacuum, respectively; (3) the dried whole bio-based PBAT, the dried lignin, a chain extender, an antioxidant, a plasticizer, a lubricant and a heat stabilizer are added into a high-speed mixing machine and fully mixed; (4) mixed materials are extruded and granulated by a twin-screw extrusion machine; and (5) lignin-PBAT composite plastic particles are subjected to film blowing to obtain the lignin modified whole bio-based PBAT biodegradable plastic is obtained . By introducing the biomass material lignin and performing compound modification, the lignin-PBAT composite plastic meets the requirements of complete degradation, and the mechanical properties of the whole bio-based PBAT resin are enhanced and the cost of the plastic is effectively reduced, thus making the plastic more conducive to market promotion.