Abstract: Modular Multilevel Converter based High Voltage Direct Current (abbr. MMC-HVDC) had been applied in various occasions. The impact of AC system short-circuit ratio and impedance angle on low frequency oscillation mode in MMC-HVDC system was researched. Firstly, an interference model of two-terminal MMC-HVDC system was established. Secondly, the impact of AC system short-circuit ratio and impedance angle on its small interference stability was researched, and the low-frequency range oscillation in MMC-HVDC system that possibly caused by weak AC grid and low impedance angle was investigated. Finally, the characteristics of low-frequency range oscillation mode was discussed, and on this basis, a regulation method of utilizing parameters of control system to restrain the low-frequency range oscillation that might appear was researched in order to provide a certain theoretical guidance for the engineering operating of MMC-HVDC system.