Abstract This study examines the changes in specimen color, surface morphology, chemical bonds, and mechanical properties of high-density polyethylene (HDPE) under the influence of aging factors such as ultraviolet light, ozone, temperature, and salt spray. The changes in mechanical properties over aging time is explained based on extensive experimental data and the findings are validated by constructing a finite element method (FEM) model. The fluctuations in mechanical properties are strongly dependent on the competition among scission, crosslinking, and rearrangement of molecular chains due to the breaking and forming of chemical bonds. The aging process is confined to the specimen surface, resulting in an aged layer hundreds of micrometers thick and slight changes in the overall mechanical properties of the specimen, as indicated by tensile test results and FEM simulations. This study will significantly contribute to understanding the aging mechanisms of HDPE and improving the durability analysis of its products.