Though there has been a large number of studies concerning the modification of wood plastic composite materials, there is still more to be done. This report aims to study the effect of the coupling agents (PP-g-MAH, silane, titanate) and compatibilizer(SBS) on the mechanical properties of straw flour/waste plastic composites. Straw flour, waste plastic, modifying agents and other additives were mixed evenly in a high speed mixing machine. And the mixed materials were compounded into the pelllets using the twin-screw extruder. Then the test specimens were prepared by the injection molding. The influence of 4% coupling agent (PP-g-MAH, silane, titanate) on the mechanical properties of straw flour/waste plastic composites was researched, and the most suitable coupling agent to the composites was obtained. In order to increase the toughness of the composite materials and to furthermore improve the comprehensive mechanical properties, the WPCs with different contents styrene butadiene styrene (SBS) were investigated. The experimental results showed that the coupling agent PP-g-MAH and the content was 4%, the mechanical properties of the composite materials were better; when SBS content was 5%, the maximum increases of tensile strength, flexural strength and impact strength of SBS modified WPCs were by 14.34%, 20.75%, 34.38% compared to those of neat WPCs respectively.