Grid-Forming converters (GFMC) considering DC-Link voltage dynamics have been widely concerned [1]. Traditional power synchronization control (PSC) based GFMC requires a constant DC-Link voltage, which is not always available. However, the existing renewable energy plants show constant power characteristics on the DC side, so the PSC based GFMC needs to be combined with the energy storage device (ESD), which will increase the operating cost of system, and the fluctuations on the DC side will seriously affect the stable operation of system. Therefore, it is necessary to introduce DC-Link voltage control (DVC) in GFMC. The commonly used cascading/parallel structure of DVC and PSC will increase the order of control loop, which results in complicated parameter tuning and stability analysis [2]. Compared with the cascading/parallel structure of DVC and PSC, DC-link voltage synchronization control (DVSC), which directly generates the synchronous frequency from DVC [3,4], is concise and has a faster control response. However, the transient stability of DVSC-based GFMC has been rarely studied. [5] established the large signal model of DVSC-based GFM-VSC. Phase portrait is adopted to analyze the influence of system parameters on transient stability. However, the phase portrait is only a qualitative analysis, and it cannot realize the effective estimation of the system's stability boundary. Moreover, A Lyapunov-based energy function is proposed in [2], which analyzed the synchronization mechanism and energy dissipation path of squared Vdc synchronization control from the view of the port-Hamilton system. However, the stability boundary is not verified, and the critical clearing angle and critical clearing time are not given. Based on the above studies, the research on transient stability analysis of GFM-VSC considering dc-link voltage dynamics is relatively insufficient. A more intuitive and accurate transient stability evaluation method is still needed. To fill this gap, an equal area criterion based on a recursive algorithm is proposed in this paper to accurately estimate the transient stability boundary of DVSC-based GFMC considering the time-varying inertia and internal voltage.