Poplar fibers were pretreated under high temperature and pressurized steam at different temperatures and pressures; they were further modified using different amounts of silane coupling agent. The obtained modified fibers were used to prepare wood fiber/high-density polyethylene (HDPE) composites by an extrusion-molding process. The effects of high temperature and pressurized steaming/silane coupling co-modification on the properties of wood fiber/HDPE composites were investigated. The results show that after the high temperature and pressurized steaming treatment, the amount of hydroxyl groups on the surface of the fibers and surface polarity of the fibers decreased, whereas the crystallinity and thermal stability significantly increased. The storage modulus and mechanical properties of the composites improved as well. The addition of silane to the steam-treated fibers further enhanced the storage modulus, loss modulus, and the mechanical properties of the composites. Moreover, when the wood fibers were only modified by adding silane and the silane content was 5%, the prepared composites showed better mechanical properties than the high temperature and pressurized steaming treatment. However, the addition of a small amount of silane to the wood fibers exposed to steam at 170 °C and 0.8 MPa provided even better results; when 2% silane was added, the composites showed the best mechanical properties.