Reactive power compensators (RPCs), as key devices for enhancing the voltage support capability of renewable energy plants, have been widely applied. However, renewable energy plants still face the risk of transient synchronization instability under large disturbances in weak grids. Therefore, to clarify the improvement effect of multi-type RPCs—including static var compensators (SVC), static var generators (SVG), and static synchronous condenser (SSC)—on the weak grid adaptability of renewable energy plants, it is imperative to investigate the mechanism by which multi-type RPCs influence the transient synchronization stability of renewable energy plants. First, considering the synchronization mechanisms and output characteristics of both multi-type RPCs and renewable energy plants, a transient synchronization model for renewable energy plants integrated with multi-type RPCs is established. Second, considering the grid Short-Circuit Ratio (SCR) and reactive power compensation capacity, the influence of multi-type RPCs on the equivalent damping, inertia, and frequency variation of the plant is analyzed; meanwhile, the improvement effect of multi-type RPCs on transient stability under weak grid conditions is comparatively studied via the phase plane method. Finally, an electromagnetic transient simulation model of renewable energy plants integrated with multi-type RPCs is built based on MATLAB/Simulink, which verifies the correctness of the theoretical analysis in this paper.