Hydration-active steel slag and slag micropowder were used as inorganic fillers with silane coupling agent (KH550) to prepare wheat straw/polyvinyl chloride wood-plastic composites (WPCs) by extrusion molding. A 35-day immersion and a pre-immersion test were carried out to analyze the influence of steel slag and slag micropowder on the physical and mechanical properties of the WPCs under wet conditions. Results showed the following: (1) KH-550 exhibited a good surface modification effect on the activated steel slag and slag micropowder, (2) an increase in the activated steel slag and slag micropowder content could effectively reduce the percent water absorption of the WPCs by 20% to 25% and the expansion by 20% to 24%, respectively, compared with the control group, but had a limited effect on the tensile strength retention, and (3) pre-immersion could effectively induce the synergistic reinforcement effect of the active fillers, resulting in reaching the saturated water absorption within 20 days. The water absorption and tensile strength were respectively 18% to 25% lower and 1.5% to 3% higher than those of the composites without pre-immersion. The results of this study could provide experimental data and theoretical references for the influence of hydration-active inorganic fillers on WPC properties.