Microplastics (MPs) are becoming a class of pollutants with high global concerns. Research on the aging of MPs has focused on oxidative environments, it is of great interest to study the aging of MPs under reduction conditions. In this study, a reduction environment was constructed by purging nitrogen and adding reducing agents (NaBH₄, VC, Na₂S, C₂Na₂O₄) to understand the aging behavior and mechanism of MPs. The results proved that PVC occurred aging under four reduction conditions, and the aging degree was the strongest under NaBH₄ reduction condition. The aged PVC became broken, particle size decreased, and dechlorination phenomenon was observed. These phenomena were more obvious under the reduction condition in light, which was the superposition of photo-aging and reduction aging. The functional group components of PVC changed (C-C/CC increased, and oxygen-containing functional groups decreased) under reduction conditions, but photo-aging was dominant in the light system. Electron transfer occurred during the reduction process, and the EDC of PVC aged increased and EAC decreased. This study may shed light on a highly efficient aging pathway of MPs that is often overlooked in nature, contributing to understanding the aging behavior of MPs in complex environments.