Going up further on the basis of excellent mechanical properties and solving the long-standing shortcoming of the slow degradation rate were developed in parallel for polylactic acid (PLA). The polylactic acid/wood flour/polymethylmethacrylate (PLA/WF/PMMA) composites were prepared by melt blending used PLA as matrix, WF of diverse particle size with PMMA at 8:2 mass ratio totally filled with 20 wt%. The PLA/WF/PMMA composites were studied by means of Fourier transform infrared spectroscopy (FTIR), differential scanning calorimeter (DSC), melting index test, and universal mechanical properties test. The result shows that WF of 150 μm < particle size < 180 μm executed the optimal improvement to mechanical properties of PLA/WF/PMMA. Tensile strength, flexural modulus and Young's modulus of PLA/WF/PMMA (150–180 μm) were enhanced 10.02%, 37.21% and 32.80% in relation to neat PLA, respectively. Hydrophilic contact angle test, water absorption measurement and hydrolysis assessment were performed to the hydrolysis degradation performance of materials. Furthermore, PMMA and WF cooperatively assisted in enormously accelerating hydrolysis of PLA, eventually having a good performance. Improving mechanical properties and accelerating degradation at the same time are of great significance for expanding the application of PLA.