Multi-infeed effective short-circuit ratio (MESCR), which can reflect strength of the AC system as well as the influence of DC systems interaction, is an important index for analysis and evaluation of multi-infeed AC/DC power system. This paper theoretically proves the limitation of the existing MESCR on evaluating the influence of massive shunt capacitor compensation, and then proposes an improved multi-infeed effective short circuit ratio (IMESCR) to take the impact of the shunt capacitors into account by converting the capacity of shunt capacitors to inverter station of HVDC. Case study is carried out based on New England 39-bus system to analyze the influence of capacity and location of the shunt capacitors on MESCR and IMESCR, respectively. The simulation results indicate that the proposed IMESCR is able to describe the strength of AC/DC power system more accurately than MESCR. Finally, the feasibility and accuracy of the proposed IMESCR are validated through simulation study of Zhejiang power grid. This work is supported by National Key Research and Development Program of China (No. 2016YFB0900805).