Subcooled boiling is a primary heat transfer mechanism in power systems such as nuclear power, aerospace, space exploration, and new energy vehicles, especially in devices like reactor cores and liquid rocket engines. Current researches on subcooled boiling mainly focuses on the heat transfer near the overheated walls and the condensation characteristics of single bubbles in the mainstream, while studies on the dynamics of multiple bubbles in the mainstream region and their impact on the two-phase heat transfer are relatively scarce. The bubble behaviors in the mainstream, such as coalescence and breakup, directly affect the void fraction, as well as the heat exchange of the mainstream fluid, thus significantly impacting the heat transfer efficiency of the system. In this study, the Volume of Fluid (VOF) method combined with the continuous surface tension model (CSF) and the Lee phase change model were adopted to numerically simulate the coaxial double bubble coalescence phenomenon during subcooled pool boiling. The results indicated that subcooling, initial bubble spacing, and bubble diameter ratio collectively determine three typical types of bubble coalescence. Detailed analysis was performed to investigate the influence of subcooling, initial bubble spacing, and bubble diameter ratio on the coalescence process, with particular emphasis on the impact of condensation-induced volume reduction on the relative motion between bubbles. Under higher subcooling conditions, it was observed that the smaller leading bubble tends to migrate toward the larger trailing bubble, moving opposing the direction of buoyancy. The simulation results reveal the phenomenon and mechanism of bubble coalescence, providing a theoretical foundation for developing a bubble coalescence efficiency model applicable to condensation phase-change conditions during subcooled boiling.