Effective short circuit ratio and critical effective short circuit ratio are common indicators for quantitative evaluation of the static voltage stability margin of a single-infeed LCC-HVDC system, but these indicators can not reflect the change of the static voltage stability of the system after STATCOM is fed in. This paper linearises STATCOM near the operating point based on the same dynamic response characteristics to small disturbances, equivalent the system containing STATCOM to a single-infeed system, specifically discusses the influence of STATCOM on the system structure parameters and state parameters combing the control mode of STATCOM in the equivalent single-infeed system and clarifies the mechanism by which STATCOM increases the static voltage stability margin of the system. Theoretical analysis and electromagnetic transient simulation results show that the static voltage stability margin of the system containing STATCOM can be accurately evaluated by comparing the relative sizes of ESCR and CESCR in the equivalent single-infeed system.