Phase transition induced by swift heavy ions in HfO₂ are detrimental to the reliability of HfO₂-based aerospace devices. In this paper, the monoclinic to tetragonal phase transition in HfO₂ has been simulated by molecular dynamics combined with the two-temperature model. The specific roles of thermal spike, oxygen defects, and their coupling effects which have been considered as the key factors contributing to the phase transition in HfO₂ were reported. Thermal spike results in sharply enhanced atomic stress, triggering the phase transition. The introduction of oxygen vacancies leads to significant reduction in the stress and corresponding electronic stopping threshold for phase transition. In addition, the presence of the oxygen defects results in reduced thermal conductivity and stronger electron-phonon coupling, which enhances the energy transmission to the lattice and further lowers the electronic stopping threshold. Sufficient number of oxygen vacancies are necessary for the survival of tetragonal phase to reduce the average potential energy of atoms.