Solvents-resistant and stable wood-plastic composites have been successfully obtained via combined torrefaction pretreatment, extrusion, and injection molding processes that applied torrefied sweet sorghum slag (SS) fibers as reinforcing fillers and high-density polyethylene (HDPE) as thermoplastic polymer matrices. Comparing with intact SS fibers, torrefaction pretreatment caused deacetylation and severe decomposition of hemicellulose (decomposed nearly 95% under 300 °C) in SS fibers, which was favorable for the formation of porous structures on the surface of SS fibers and the increase in their hydrophobicity. Thus the resultant composites exhibited enhanced mechanical properties (max tensile strength of ~24.84 MPa, flexural strength of ~32.63 MPa, and toughness of ~2.50 MJ/10−3), and promising solvents resistance (e.g., H2O, acid, alkali, salt, and ethanol) and stabilities (e.g., thermal, and dynamic mechanical stabilities). The mechanical strength retention rates of HDPE/torrefied SS (300 °C) composites maintained above 95% after socking in common solvents for 7 days. Moreover, torrefaction took advantages of short processing time, energy-saving, low-cost, and high mass yield when compared to the traditional carbonization, which paved an efficient and scalable biomass fibers pretreatment strategy to construct highly stable wood-plastic composites and extended their application areas like constructing materials and some extreme circumstances.