Abstract: Polylactic acid (PLA) is a green, renewable and degradable polymer material, which is considered to be the most commercially available material at present. However, the rigidity of the main chain structure of PLA material determines its weak impact strength and low elongation at break, which limits its application in a wide range of fields. In this study, a twin-screw extrusion method for preparing PLA and polyethylene glycol (PEG) modified lignin biocomposites was proposed, in order to improve the mechanical properties of 3D printed lignin/PLA composites and expand the application field. The structure of PEG modified lignin was studied. The effects of soda lignin and PEG modified lignin on the physical and mechanical properties, microstructure, thermal properties and degradation properties of PLA composites were studied. The experimental results showed that PEG can be grafted onto lignin and the thermal properties of lignin can be improved. The addition of PEG modified lignin increased the heat resistance of PLA composites. Compared with L30/PLA, the tensile stress and elongation at break of PL30/PLA were increased by 18.1% and 81.9%, respectively. Compared with pure PLA, the composite has a better degradation rate in water systems, especially in alkaline media. Therefore, PEG modified lignin not only improved the added value of lignin, but also provided a new way for the production of high-performance PLA composites.