The invention relates to composites and methods for producing a plastic-steel square timber and a straw foamed plastic square timber. The composite for producing the plastic-steel square timber comprises the components of PVC (polyvinyl chloride), light calcium carbon powder, a PVC stabilizer, a PVC foaming regulator, an AC (azodicarbonamide) foaming agent, a stearic acid, PE (polyethylene) wax, ACR (acrylate), titanium dioxide, calcium stearate and DOP (dioctyl-phthalate). The composite for producing the straw foamed plastic square timber is prepared from straw power in addition to the components of the composite for producing the plastic-steel square timber. The methods for producing the plastic-steel square timber and the straw foamed plastic square timber from the composites comprise the following steps: proportioning materials, blending materials, adding materials, extruding, molding, cooling by water, quickly freezing, dragging and forming. The plastic-steel square timber and the straw foamed plastic square timber can replace the original wooden timber, are made of single materials, are convenient to product and recycle and can be smashed for secondary machining and shaping.