Novel thermoplastic composites filled with wheat straw (WS) and enzymatic-hydrolysis lignin (EHL) were developed and characterized. The three-dimensional melt blending system of WS, EHL, and high-density polyethylene (HDPE) was optimized via orthogonal experiments. The mechanical properties and melt index of the composites were measured and the optimum ratio of the composites was determined. Based on the optimum ratio of the composites’ blending system, identified through compounding granulation and extrusion molding process links, pilot products of the composites were produced. The thermal behavior, polar groups, and surface structures of the fibers and developed thermoplastic composites were assessed by thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR), and scanning electron microscopy (SEM) analysis, respectively. The addition of EHL, an abundant renewable resource, improved the dispersity of the matrix as well as the mechanical and thermal properties of the composites. The results provide a theoretical basis for the application and development of new composites and illustrate a potential industrial application of EHL.