Abstract: When the short-circuit ratio of the renewable energy long-distance transmission system is relatively low, it is easy to cause the problems such as oscillation and overvoltage. So there is an urgent need to improve the power grid strength of the sendingend power grid. In view of the advantages of the synchronous condenser（SC） in the dynamic reactive power support, the industry generally takes measures to install the SC to improve the transmission capacity of large-scale renewable energy bases. However,the SC is a synchronous generator intrinsically, therefore there is a risk of transient power angle instability especially when the short-circuit ratio is low and the control strategies of renewable energy do not match well. This potential problem has not attracted enough attention from the industry. To this end, the power angle characteristics of the SC of the renewable energy transmission system are firstly analyzed, and the transient power angle instability mechanism of the SC is analyzed. Secondly, the equivalent convert coefficient of the renewable energy capacity and the critical short-circuit ratio condition of the system are proposed to quantify the synchronization stability of the SC. Furthermore, the key factors affecting the power angle stability of the SC are analyzed, which provides a theoretical basis for the subsequent coordinated control strategy to improve the synchronization stability margin of the SC. Simulation examples verify the effectiveness of the proposed stabilization mechanism and influencing factors.