The flexible control characteristics and topology of VSC-MTDC system, which inserts into the AC system, will change the fault response of the AC/DC hybrid system. This paper aims at studying the effect of VSC-MTDC system of different topological structures and different control modes on power system transient stability. In this paper, firstly, a four-terminal flexible HVDC transmission system is constructed. The statistical analysis of clearing time critical with different topologies in three phase short circuit fault is carried out. Through statistical analysis, the relationship between VSC-MTDC topology and transient stability is obtained. Based on that, studying the influence of control mode on transient stability, in turn, four converters are set to the constant DC voltage control, analysis of the impact of control on the stability of the power grid integrated proximal and distal line fault.