Abstract: Lithium-ion batteries, which are widely used energy storage devices in modern electronic devices and electric vehicles, have been identified as a key component in the transition from depleted fossil fuels to sustainable energy production and use worldwide. However, due to misuse and improper use, lithium-ion batteries can experience thermal runaway, a phenomenon that manifests itself as a significant build-up of gases within the battery. Traditional solid-liquid two-phase models present challenges in characterizing the complexity of thermal runaway mechanisms. In order to understand the mechanism of thermal runaway more accurately, an electrochemical-thermal coupling model of 18650 lithium-ion battery pack is built based on gas-liquid two-phase flow, and the variation law of internal stress, temperature and electrolyte concentration of lithium-ion battery during thermal runaway is well studied. According to the chemical reaction process and the dynamic evolution of internal pressure and temperature, the whole thermal runaway process is divided into reaction stage, diffusion stage and equilibrium stage, so that the characteristics of each stage can be studied in more detail. The effectiveness of the theoretical analysis is verified by using the COMSOL Multiphysics 5.5 software, and the theoretical model is modified, and the results of the comparative analysis show the importance and complementarity of the theoretical analysis and simulation experiments in the study of thermal runaway of lithium-ion batteries. The difference between the peak value of the internal stress correction value and the simulated value is only 0.04 MPa, the error is significantly reduced, and the accuracy is increased by 80%, which indicates that the model can more accurately describe the thermal runaway process of lithium-ion battery, highlighting the accuracy enhancement obtained by this modeling method. Our study also reveals that the thermal runaway gas can cause a drastic change in the internal temperature of the battery, and the amplitude of the change is basically synchronized with the increase of the internal pressure of the battery, the thermal runaway occurs at a temperature of 60 ℃, and the thermal runaway gas is ignited when the internal temperature rises to 160 ℃, and the maximum simulated temperature and corrected temperature are 177 ℃ and 172 ℃ at 640 seconds, respectively. The formation of hot spots is the main cause of uneven temperature distribution within the battery. In addition, Our study shows that the electrolyte concentration decreases from 1 200 mol/m3 to 837 mol/m3 simultaneously from the theoretical and simulated values of the electrolyte concentration caused by the thermal runaway gas throughout the thermal runaway process. This dynamic evolution trend accelerates the formation of gas-liquid two-phase flow and the process of thermal runaway, forming gas-liquid two-phase flow, the interaction between the electrolyte and the electrode material, which leads to the reduction of the electrolyte concentration to a large extent. The electrochemical-thermal coupling model based on gas-liquid two-phase flow and the in-depth study of the thermal runaway process deepens our understanding of the mechanism of thermal runaway of lithium-ion batteries, and provides a more in-depth theoretical basis for preventing or remedying the thermal runaway problem of lithium-ion batteries.