The work aims to discuss the effects of particle size and content of cassava straw powder on the physical and mechanical properties and interfacial bonding properties of the composites, so as to improve the utilization of waste cassava straw. Wood plastic composites were prepared with cassava straw powder as reinforcement, high density polyethylene (HDPE) as matrix and maleic anhydride grafted polyethylene (MAPE) as coupling agent. The tensile properties, bending properties, notched impact strength and water absorption of wood plastic composites were tested, and the cross-section microstructure of the composites was observed and analyzed by the electron microscope (SEM). With the increase of the content of straw powder, the tensile strength and bending strength on the whole showed an increasing trend, with the maximum values reaching 32.5 MPa and 49.6 MPa respectively, while the notched impact strength kept decreasing. When the particle size decreased, the tensile strength of the material firstly decreased and then increased, the bending strength had little difference and the notch impact strength decreased as a whole. When both the content of straw powder and the particle size decreased, the composites showed better water resistance. When the mass fraction of the powder is 60% and the particle size is 40～60 mesh, the composites have excellent comprehensive properties, and the relevant properties exceed the standard value of wood plastic decorative board (GB/T 24137—2009).