Abstract: To reveal the long-term tensile properties of polymethyl methacrylate (PMMA) in a water environment, water-soaking and temperature-accelerated aging tests as well as tensile mechanical property tests were conducted. PMMA specimens were immersed in water at 30, 50, and 70 ℃ for 30, 60, and 90 days, respectively, and the tensile strength of PMMA under each temperature and immersion duration was tested. The results showed that an increase in temperature significantly accelerated the degradation of tensile strength of the specimens. After 90 days of aging, the tensile strength retention rate of the specimens at 70 ℃ was the lowest. Overall, the yield stress, tensile strength, and fracture strain of the specimens decreased with the extension of aging time and the increase in temperature. In this study, two methods were used to establish a long-term performance prediction model for PMMA: the time–temperature superposition (TTS) principle and the Arrhenius chemical reaction rate theory. The former constructed a master curve of tensile strength retention rate-time to extrapolate long-term performance, while the latter established a temperature extrapolation model for the degradation rate based on thermal activation kinetics. The predicted tensile strength retention rates for a service life of 25 years at 20 ℃ were approximately 0.92 and 0.88 for the two methods, respectively. It is recommended to prioritize the Arrhenius method for long-term performance prediction of PMMA in a water environment, with the TTS method used as a supplementary reference.