Two-phase closed thermosyphons are widely used as efficient devices for heat transfer and have undergone significant development over the past several decades due to ultra-high equivalent heat transfer coefficients. Considering that experimental research requires significant time and resources, computational fluid dynamics methods are generally used by scholars to predict the vapor−liquid two-phase flow mechanism and performance of a two-phase closed thermosiphon based on its reliability. In this study, a dynamic Lee model coupled with a volume of fluid model and continuum surface force model was developed. The modified model was used to investigate the heat transfer characteristics, vapor−liquid phase change process, and details of the two-phase flow during the operation of a two-phase closed thermosyphon. In the modified model, dynamic adjustment of the mass transfer time relaxation parameters for the Lee phase change model is realized. The values of the mass transfer time relaxation parameters and amount of vapor−liquid mass transfer stabilize after 4 s of the update iteration calculation. The relative errors between the modified model and experimental data for the temperature distribution and total thermal resistance are 5% and 19.0%, respectively, representing acceptable agreement. These results indicate that the modified model can ensure good vapor−liquid mass balance characteristics and has good accuracy.