Abstract: To address the thermal safety management of power lithium-ion battery modules for electric vehicles, a composite thermal runaway （TR） suppression strategy utilizing immersion cooling synergized with thermal insulation materials was proposed. Experiments were conducted to quantitatively investigate the temperature rise characteristics, mass loss, and thermal propagation behavior of modules under both immersed and non-immersed conditions. The results demonstrate that, compared to non-immersed cooling, the composite immersion cooling strategy reduces the peak module temperature by 25.3% and decreases the mass loss rate by 13.9 percentage points. Furthermore, the TR propagation rate to adjacent cells under composite cooling is reduced by 24.3% compared to that of the non-immersed cooling with 1mm thick aerogel insulation. However, combustion of the immersion fluid （polyolefin oil） significantly reduces the system's thermal safety margin, with the jet flame height increasing by 3.6 times compared to that of the non-immersed scenario. Under the composite cooling scheme, the peak local temperature rise rate within the module decreases by about 70%. Thermal decomposition of the insulation material in high-temperature environments leads to performance degradation in long-term insulation effectiveness. The outermost cell exhibits notable self-attenuation characteristics during TR, with its mass loss reduced by 16.7% compared to that of the targeted TR cell.