High-scalability HfO₂-based ferroelectric thin films are promising for application in fast, energy-efficient, and high-density non-volatile memories. This ferroelectricity is believed to originate from the metastable orthorhombic phase, which is difficult to obtain. Post-metallization annealing with a top electrode capping layer is a useful method for stabilizing the ferroelectric orthorhombic phase. However, direct physical evidence of the top electrode role is lacking. In this study, we visualized the dynamic process of the phase transition in Hf_0.5Zr_0.5O₂ (HZO) thin films with TiN and Pt top electrodes during the heating and cooling processes through in-situ scanning transmission electron microscopy (STEM). The TiN top electrode stabilized the orthorhombic phase, whereas the Pt top electrode induced a phase transition to the monoclinic phase. Subsequently, we elucidated the phase transition mechanism in HZO thin films using the kinetic effect and revealed that it was related to the concentration of oxygen vacancies induced by the top electrode. This study provides valuable insights into the stabilization of the orthorhombic phase in HfO₂-based ferroelectric thin films and contributes to the elucidation of the phase transition mechanism of HfO₂-based ferroelectric thin films.