Flax fibers with a mass fraction of 0~40% were incorporated into polylactic acid (PLA) composites. Based on the pre-mixing of grinding extrusion (GE) and internal mixing (IM) processes, injection molding was used to prepare flax fiber/polylactic acid (PLA) composites. The results show that the incorporation of flax fibers can significantly improve the mechanical properties and biodegradability of the composites, with no significant effect on thermal stability. The composites obtained by the IM process have more excellent mechanical properties and biodegradability than those obtained by the GE process. The 40%L/PLA-IM composite has the optimal mechanical properties and biodegradability. The tensile strength is 63.5 MPa, the elasticity modulus is 2.56 GPa, the flexural modulus is 3.49 GPa, the impact strength is 5.58 kJ/m², the water absorption rate after 20 days of degradation is 126%, the weight loss rate after 20 days of degradation is 84.1%, and the lactic acid production after 20 days of degradation is 36.2 mL. The preparation of flax fiber/PLA composites using the IM process is feasible.