Abstract: Currently, accelerated aging tests are widely used to study the aging process of polyethylene pipelines. However, this approach can only simulate one or several main influencing factors in the natural environment, which are often quite different from the actual environment of the buried pipelines. In this study, five types of PE80 buried pipelines in service for 9–18 years were taken as the research object, while new PE80 pipelines were taken as the reference group. The aging process and mechanism of polyethylene buried pipelines were studied through mechanical and chemical property tests and microstructural analysis. The results showed that the pipeline exhibited cross-linking as the main aging mechanism after being in service for 0–18 years. The aging degree and law of the inner and outer surface of the pipeline were compared, and the observed mechanism of both surfaces was explained. After 18 years in service, the elongation at the break of the pipe decreased by 16.2%, and the toughness of the matrix in the main collapse area of the tensile sample was the fundamental reason responsible for changes in the mechanical properties. Finally, after 18 years in service, the oxidation induction time of the pipeline was 25.7 min, which was 28.5% higher than the national standard value. There were no potential safety hazards during continuous long-term service. The results of this paper provide reference data and theoretical guidance for the aging process study of buried polyethylene pipelines.