To assess the risk of thermal runaway in 21700 ternary lithium-ion batteries, a comprehensive quantitative risk evaluation was conducted using AHP (analytic Hierarchy process) method, focusing on both the sensitivity and severity of thermal runaway. This evaluation was based on a lithium-ion battery thermal runaway experimental platform, which collected data on battery surface temperature, mass loss, combustion and explosion pressure, and CO/CO₂ concentrations under various heating power and SOC conditions. The results indicate that as SOC and heating power increase, the safety valve opening temperature (T_v) and thermal runaway trigger temperature (T_TR) gradually decrease, while the time lag (Δt) and temperature difference (ΔT) between safety valve opening and the onset of thermal runaway significantly shorten. Additionally, mass loss, peak combustion/explosion pressure, and CO/CO₂ concentrations increase markedly. CHI (comprehensive hazard index) increases monotonically with SOC and heating power. The high-risk zone is primarily concentrated in operating conditions where SOC≥75% and heating power ≥150 W. The medium-risk zone corresponds to operating conditions with SOC between 50% and 75% or high SOC but low heating power; and the low-risk zone primarily consists of operating conditions where SOC≤25% and heating power ≤150 W. The safety risks of lithium-ion batteries in enclosed environments can be assessed with the aid of this study, and foundational data and support are provided for the safety of lithium-ion battery energy storage and transportation, as well as for battery safety design.