Despite the growing deployment of photovoltaic systems in arid regions, the degradation behavior of polymeric backsheets under arid-cold climates remains insufficiently understood. This study investigates the long-term aging of poly (ethylene terephthalate) (PET) backsheets exposed outdoors in Yuli (located in Xinjiang, China), a representative arid-cold environment. A comprehensive suite of analytical methods including NMR, GPC, FTIR, 2D-COS, mechanical testing, and other techniques was employed to characterize the physicochemical and structural changes. Upon aging up to 450 days, PET backsheet film exhibited pronounced mechanical embrittlement, discoloration, significant surface crystallization, and enhanced oxygen barrier properties. These changes are primarily attributed to UV-driven chain scission and progressive methylene oxidation. The results from Yuli were further compared with those from Chengdu (a typical humid subtropical climate) and from standardized xenon-arc and damp-heat tests. The extent and characteristics of acid accumulation were found to vary with climate; xenon-arc exposure induced rapid acid generation accompanied by ester degradation, whereas damp-heat aging produced only minimal detectable acids. Chengdu samples show intermediate behavior with sustained acid development. These findings reveal the limitations of existing standard protocols and offer mechanistic insights that can inform the design of climate-adapted accelerated aging protocols for evaluating photovoltaic reliability.