To achieve rapid bonding with desired adhesion properties, a combined treatment of sanding then coating with polyisocyanate, followed by plasma discharge was implemented on the surfaces of polyethylene wood-plastic composites (WPCs). The surface properties of polyethylene WPCs were studied by evaluating the contact angle and bonding strength, as well as analyzing it via Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The results indicated that the shear strength and durability of the bonding joints of polyethylene WPCs increased considerably due to the synergetic effect from the surface treatment. Thus, a rapid bonding with desired adhesion properties of polyethylene WPCs was achieved. The roughness and the oxygen content on the composites’ surface increased after the combined treatment. Polar functional groups, such as –OH, –C=O, and –O–C=O, formed on the surface. At the same time, the presence of –NCO and –NH functionalities, generated during the combined treatment, showed that chemical bondings between polyisocyanate and the wood fibers of the composites occurred. The changes on the surface properties, such as roughness, wettability, as well as formation of chemical groups, substantially affected the adhesion properties of the bonding joint for polyethylene WPCs.