Abstract: The development of steel slag/stone-wood plastic composites reinforced with calcium sulfate whisker is beneficial for reducing costs in the stone-wood plastic industry and promoting the resource utilization of industrial waste. Steel slag powder (SSP) composited with calcium sulfate whisker (CSW) was investigated as a replacement for a portion of talc powder (TP) in the creation of calcium sulfate whisker-reinforced steel slag/stone-wood plastic composites (CSW-SSP/SPCs). The reinforcement effect and thermal stability mechanism of CSW within these composites were examined by assessing their mechanical properties, mineral composition, structural composition, thermal stability, crystallinity, and microstructure. The results showed that the tensile strength, flexural strength, and impact strength of CSW-SSP/SPCs were increased by 28.13%, 25.02%, and 45.55%, respectively, which were significantly better than those of the pure TP sample. The SSP composited with CSW effectively replaced part of the TP, where CSW significantly reinforced the composites through its bridging, micro-filling, and synergistic effects with the SSP. Meanwhile, the MgO, Al₂O₃, and Fe₂O₃ in the SSP crosslinked with the carbon layer skeleton and residual materials to form a more stable carbon layer, which inhibited the combustion reaction and further enhanced the thermal stability and retarded the thermal degradation process.