Wood plastic composites (WPC) are a new type of green environmental composites with high performance and added value. In this paper, poplar wood flour was modified with vinyltrimethoxysilane(A-171) to improve the interfacial compatibility and enhance the interfacial bonding between the polar wood flour and nonpolar plastics. Then the modified wood flour was blended with high density polyethylene (HDPE) and prepared composites by extrusion. The effects of amount and solution concentration of A-171 on the properties of WPC were investigated. FTIR analysis indicated that polycondensation grafting reaction occurred between A-171 and wood flour. Compared to untreated wood flour/HDPE composites, both mechanical properties and water resistance of the composites were improved and presented optimal performance when the dosage of A-171 was 4%. The storage modulus G' and viscosity η*of the composites decreased but the loss tangent tanδ increased compared to the untreated composites. G' and η* raised with the increase of A-171, however, tanδ decreased. Solution concentration of A-171 did not show significant effect on the mechanical properties of composites, but water resistance of composites decreased with the increase of solution concentrateon of A-171.