Thermal runaway (TR) is a critical safety concern in energy storage batteries, and the effects of aging on battery TR characteristics are of utmost importance. In this study, prismatic lithium-ion batteries with a nominal capacity of 40 Ah and Li(Ni0.6Co0.2Mn0.2)O2 cathode material were subjected to cycling aging up to 80% of their rated capacity. The TR characteristics of the aged batteries were investigated at different state of charge (SOC) levels, including 25%, 50%, 75%, and 100%, in an inert environment. The findings revealed that as SOC increased, various parameters, such as the maximum temperature of the safety valve outlet (TE,max), the highest temperature on the battery surface (TS,max), the highest temperature in the experimental chamber (TC,max), the mass loss rate, the highest temperature at the positive electrode tab (TP,max), and the highest temperature at the negative electrode tab (TN,max), all exhibited an upward trend. Additionally, higher SOC levels resulted in higher total gas production, while normalized gas production showed a decreasing trend. These results provide valuable insights for early warning of thermal hazards and internal thermal state analysis of batteries, which can inform the design of energy storage batteries with improved thermal safety measures.