Abstract: Molecular dynamics simulations were performed to investigate the effects of size difference and helium-to-vacancy number ratio (n_He/n_V ratio) on the helium bubble coalescence and hydrogen-helium bubble coalescence in BCC iron. The role of hydrogen in hydrogen-helium synergistic effects on bubble coalescence was clarified by comparing the process of helium bubble coalescence and hydrogen-helium coalescence. The results show that, in addition to the two situations where two neighboring helium bubbles or hydrogen-helium bubbles coalesce and do not coalesce, there exists an additional situation where the coalescence process terminates after a certain number of helium atoms are exchanged. As the size difference between the two helium bubbles or hydrogen-helium bubbles involved in the coalescence process decreases, the probability of helium atom exchange or coalescence process between helium bubbles and hydrogen-helium bubbles both increases. With the increase of the initial n_He/n_V ratio, the two bubbles can coalesce with each other. Moreover, the greater the size difference, the higher the required n_He/n_V ratio for bubbles to coalesce. Hydrogen-helium bubbles can undergo helium atom exchange or even coalescence processes under conditions of larger size difference compared to helium bubbles. Furthermore, the n_He/n_V ratio required for the coalescence of hydrogen-helium bubbles is lower than that of helium bubbles. These results indicated that hydrogen plays a role in promoting the hydrogen-helium bubble coalescence process in the hydrogen-helium synergy effects.