For multi-infeed HVDC systems, when analyzing the high-frequency resonance risk of voltage source converter-high voltage direct current （VSC-HVDC） and AC external systems, the influence of other DC in the near region on the AC external impedance characteristics is often overlooked. To assess the high-frequency resonance risk between a single VSC-HVDC and external system, this paper first introduces a calculation method of harmonic impedance of AC external system including multi-infeed DC and other power electronic equipment. Considering the problems of multiple DC operation modes, multiple parameters, complex acquisition of harmonic impedance characteristics, low efficiency and questionable necessity of detailed calculation in the system, the multi-infeed interaction factor （MIIF） index is introduced. The MIIF measures the strength of interaction between various converter stations in the multi-infeed HVDC system to evaluate the DC that needs to be considered in detail when calculating the harmonic impedance of VSC-HVDC external systems, This allows for fast and accurate calculations of the harmonic impedance of VSC-HVDC external systems. Finally, based on the actual data of a multi-infeed HVDC provincial power grid, the accuracy of the proposed method is verified. The proposed method provides a more accurate, fast and effective method for high-frequency risk analysis and avoidance of VSC-HVDC planning operation in multi-infeed HVDC system.