Abstract: The AC system and control system parameters have different effects on small-signal stability when modular multilevel converter（MMC） operates in the rectification and inversion modes. Firstly, a small-signal model, including MMC station, AC system and control system, was developed, and was validated by the detailed electromagnetic transient（EMT） simulation in PSCAD/EMTDC. Then, based on the eigen-analysis and participation factor method, the effects of short circuit ratio（SCR）, impedance angle of AC system and control system parameters on the small-signal stability of the MMC system in the rectification and inversion modes were compared. The results show that with the increase of the impedance angle, the stability margin of the MMC in the rectification mode is enhanced, however gets worsen for the MMC in the inversion mode. In the weak AC system, the phase-locked-loop（PLL） parameter has a greater effect on the small signal stability of the MMC system in the inverter mode. In addition, the feasible region of the circulating current suppression controller（CCSC） parameters of the MMC in the rectification mode is much larger than that in the inversion mode. This work can provide valuable theoretical guide for system design and control system selection of MMC-HVDC projects.