Abstract: Internal short circuit of lithium dendrites is one of the main causes of thermal runaway in lithium-ion batteries for new energy vehicles. Understanding the characteristics of lithium-ion battery dendrites and temperature is beneficial for reducing the risk of internal short circuit thermal runaway in lithium batteries. In this paper, a model of electrochemical-thermal internal short circuit in lithium batteries is established, the simulation calculation time of internal short circuit development is 0.01 s, and the influence of factors such as the radius of lithium dendrites and the solid phase volume fraction of battery electrodes on the electrical and temperature distribution characteristics of the battery has been analyzed. The results show that the initial temperature of the internal short circuit, short-circuit current, and heat generation power increase with time. The internal short circuit temperature and lithium dendrite radius show a quadratic positive correlation, a linear relationship with volume fraction, and a negative correlation with membrane thickness and thermal conductivity. After the internal short circuit occurs, with the removal of temperature restrictions at the electrode, the initial temperature of the internal short circuit begins to rise over time. The time to reach the starting temperature of thermal runaway for lithium dendrite radii of 5 μm and 10 μm is 4 s and 1.8 s, respectively. This study provides data support for temperature detection and fault diagnosis during the operation of lithium-ion batteries, and guidance for hazard investigation and daily prevention in the use of new energy vehicles.