Multi-infeed effective short circuit ratio (MIESCR)is a common index to evaluate the voltage supporting capacity of AC power grid in multi-infeed systems, but its physical significance and applicable scope are not clear enough. In this paper, the multi-infeed system is equivalent to a single-infeed system with the same external characteristics, and the definition of the generalized multi-infeed equivalent effective short-circuit ratio (MEESCR)is proposed. It is proved that effective short circuit ratio (ESCR)and MIESCR are special cases of MEESCR under certain operating conditions after some approximations, and the applicable scope and the error source of MIESCR and ESCR are explained theoretically. The mechanism analysis and PSCAD simulation results show that MEESCR can more accurately evaluate the voltage support capability of AC grid in multi-infeed system compared with MIESCR and ESCR.