Poplar biochar was prepared and utilized to produce composites based on HDPE. It was found that with the increase of pyrolysis temperatures, the number of polar functional groups decreased, and the specific surface area and pore volume of PB increased, which provided the possibility for the preparation of composites. The addition of PB had no effects on the microcrystalline structure of HDPE, but the increment of PB could reduce the intensity of the crystalline peaks of the composites. There were no chemical reactions between PB and HDPE, PB did not change the melting temperature of HDPE, but the increment of PB could promote the early crystallization of the composites. The interlocking structure between PB and HDPE improved the flexural strength of the composites, but the addition of PB could reduce the toughness of HDPE, and the increment of PB had negative effects on the impact strength of PB/HDPE composites.