A dissipative particle dynamics simulation method is used to investigate the vesicle fusion behavior in this study, especially to probe the driving force for vesicle fusion. Based on the former study, the fusion mechanism is investigated from the point of conservative energies. It is found that the total energy is decreasing during vesicle fusion, which is mainly caused by the reduction of conservative energy. Through further exploring the six interaction energies of particle-pairs, it is demonstrated that the energy reduction is attributed to the interaction energy between water and amphiphiles. The pressure variation confirms that vesicle fusion is a process of energy releasing.