The application of wood-plastic composites is growing rapidly in the fields of corrosion and aging. The present study investigated the effect of alkali-treated (with NaOH concentrations of 0.5, 2.5, 4.5, and 6.5%) sorghum straw (SS) fiber on the wear resistance of polyvinyl chloride (PVC) composites under simulated seawater and acid rain conditions. The results showed that the wear resistance of the SS/PVC composites was noticeably improved by the addition of the alkali-treated SS fiber. SS fiber treated with 4.5% NaOH showed low polarity and hydrophilicity with high crystallinity and improved mechanical properties, which endowed the SS/PVC composites with high interfacial bonding and wear resistance. Exposure to simulated seawater and acid rain resulted in the deterioration of physical (including water repellency and hardness), mechanical, thermal, and wear-resistance properties of the composites; the wear mechanism of the SS/PVC composites after corrosion for 12 days was mainly abrasive wear.