Synchronous stability is crucial for the safety and operation of AC power systems. However, most of the current researches focused on the stability of grid-connected converters, and that of renewable equipment still lacked. In this paper, the impact of the additional inertia control (AIC) on the permanent magnet synchronous generator (PMSG) is studied. It is found that with the AIC, the system dominant dynamics shifts from the electromagnetic to electromechanical timescales. This paper develops a simplified model for the single-PMSG infinite-bus system with the AIC within the electromechanical timescale, and reveals the transient synchronization stability mechanism from three aspects: the machine-network interface, transient dominant variable, and interaction between the synchronous loop and the power imbalance loop. Finally, this paper analyzes the swing characteristics of the PMSG system, and uncovers the relationship between the energy transmission and synchronization. These findings could provide improved insights into the synchronous stability mechanism of renewable-dominated new-type power systems.