Abstract: WF/HDPE composites were prepared by melt extrusion with poplar fiber (WF) as reinforcing material, high density polyethylene (HDPE) as matrix and maleic anhydride grafted polyethylene (MAPE) as coupling agent. The addition amount of WF, the addition amount of coupling agent and the extrusion temperature were selected as independent variables, and the impact strength, bending strength and tensile strength of the specimen were selected as the response value. The experiment was designed by Box-Behnken Design method and the secondary mathematical model of mechanical strength of WF/HDPE composites was established by the response surface method to optimize the extrusion process of the composites. The results show that the optimum levels of WF addition, MAPE addition and extrusion temperature are 47.37wt%, 4.23wt% and 173.69℃, respectively. The corresponding impact strength, bending strength and tensile strength of WF/HDPE composite are 4.06 kJ·m⁻², 43.79 MPa and 28.59 MPa. The error between the predicted value of the model and the measured value is less than 5%, which well reflects the relationship between the mechanical properties and the factors of extrusion process of the WF/HDPE composites.