Abstract: Rice husk biochar and poplar biochar were prepared from rice husk and poplar at 600 °C in this study. Rice husk, rice husk biochar, poplar and poplar biochar were used to reinforce high density polyethylene (HDPE) to prepare composites, and properties of the composites were tested and analyzed respectively. The results show that higher carbon content, higher specific surface area and more developed pore structure are obtained in rice husk biochar and poplar biochar than rice husk and poplar. The flexural strength, flexural modulus, tensile strength and tensile modulus of rice husk biochar/HDPE composites are 34.95 MPa, 1.76 GPa, 26.25 MPa, 1.83 GPa, higher than those of rice husk/HDPE composites. Similarly, the flexural strength, flexural modulus, tensile strength and tensile modulus of poplar biochar are 40.14 MPa, 2.43 GPa, 30.64 MPa, 2.17 GPa, higher than those of poplar/HDPE composites. Besides, the creep resistance and anti-stress relaxation ability of rice husk biochar/HDPE composites and poplar/HDPE composites are also stronger than rice husk/HDPE composites and poplar/HDPE composites. The results of this study suggest that the mechanical properties of biochar/HDPE composites are better than biomass/HDPE composites, which can provide new ideas for the high value utilization of agroforestry wastes.