Lithium-ion cells normally operate during 0% and 100% state of charge (SOC), therefore thermal runaway can occur at any SOC. In this paper, the 74 Ah lithium-ion pouch cells with the Li(Ni0.8Co0.1Mn0.1)O2 cathode were thermally abused by lateral heating in a semi-open chamber. The differences of thermal runaway behavior were investigated under six SOCs. Characteristic parameters such as triggering time and triggering temperature for thermal runaway show a negative correlation with SOCs, while maximum surface temperature and maximum surface temperature rise rate show a strongly positive correlation. Besides, mass loss ratio increases exponentially with equivalent specific capacity statistically, which implies that the pouch cells with high specific energy density and high capacity may eject more violently. Furthermore, the impact on the surroundings caused by high-temperature ejections was studied, and maximum ambient temperature and maximum ambient pressure in the chamber reached a plateau at middle SOCs. Based on the thermal impact on the surroundings, a theoretical method is proposed to evaluate the deterioration of heat dissipation by venting, and simplified to quantitatively calculate the deterioration under above SOCs. The results can provide guidance for battery safety management strategies and structure design of the battery pack.