The thermal safety performance of lithium-ion batteries is significantly affected by high-temperature conditions. This work deeply investigates the evolution and degradation mechanism of thermal safety for lithium-ion batteries during the nonlinear aging process at high temperature. Through a comprehensive analysis from multiple perspectives, it has been revealed that lithium plating and R-H+ reduction are the primary factors contributing to the notable deterioration for battery safety performance during high-temperature aging. Side reactions, such as lithium plating and R-H+ reduction, occurring with aging, not only result in the thickening of the interfacial film but also generate a significant amount of reductive gas. Consequently, these results in a significant increase in cell impedance and a notable decrease in cell thermal stability. These findings offer valuable insights for the development and optimization of safer cell designs.