In order to analyze the receiving-end AC side's harmonic interaction mechanism of the converter station in the hybrid multi-infeed HVDC transmission system, the harmonic impedance model of hybrid multi-infeed HVDC transmission system is established. Harmonic multi-infeed interaction factor (HMIIF) is proposed to measure harmonic interaction. The results of analysis based on the harmonic impedance matrix show that, the law of harmonic interaction between LCC-HVDC and MMC-HVDC is basically the same as that between different LCC-HVDC systems. The harmonic interaction between LCC-HVDC and MMC-HVDC changes with the harmonic impedance of MMC's converter in the AC side and it increases with the decreases of the coupling impedance between DC placements; the harmonic interaction of other DC systems to a certain DC decrease with the growth of the DC's equivalent harmonic impedance of AC subsystems; the harmonic interaction of this DC system to other DCs increases with the growth of the DC's equivalent harmonic impedance of AC subsystems. The index HMIIF and harmonic interaction between two DC systems are inversely proportional relationship. Finally, The simulation results of a hybrid three-infeed HVDC transmission test system built by PSCAD/EMTDC verify the correctness of the conclusions.