To assess the risk of thermal runaway in 21700 ternary lithium-ion batteries, a comprehensive quantitative risk evaluation was conducted using the Analytic Hierarchy Process (AHP) 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/CO? concentrations under various heating power and SOC conditions. The results indicate that as SOC and heating power increase, the safety valve opening temperature (Tv) and thermal runaway trigger temperature (TTR) 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/CO? concentrations increase markedly. The Comprehensive Hazard Index (CHI) 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. This study aids in assessing the safety risks of lithium-ion batteries in enclosed environments, providing foundational data and support for the safety of lithium-ion battery energy storage and transportation, as well as for battery safety design.