Aiming at the shortcomings of the traditional multi-infeed effective short-circuit ratio that cannot account for the dynamic response process of STATCOM, this paper proposes a modified multi-infeed effective short-circuit ratio based on Thevenin’s dynamic equivalence method, taking the impact of STATCOM into account. Such index can be obtained by working out the system equivalent impedance with BPA dynamic simulation, which will better describe the strength of the receiving-end system. Moreover, according to the proposed modified multi-infeed effective short-circuit ratio, aiming at improving the safety and stability of the receiving-end AC/DC hybrid grid, this paper quantifies the reactive power compensation effect of STATCOM from two aspects of totality and balance, a complete set of STATCOM capacity-determine method is established. Finally, the effectiveness of the proposed index and method are verified by an actual grid calculation example.