Abstract: The control loops of grid-forming（GFM） converters affect the low-frequency oscillation（LFO） mode of the AC system, and the influence mechanism of different GFM control strategies are obviously different. Focusing on the modular multilevel converters（MMCs）, this paper establishes a damping torque analysis model for the paralleled system with synchronous generators and GFM converters, to study the impact of different GFM control strategies on the LFO of AC systems. Based on the model, this paper studies the influence of typical control loops of the converters using virtual synchronous generator（VSG） control, droop control, and magnitude-phase control on the damping torque component of the system. Meanwhile, the oscillation instability mechanism induced by the dominant control loops is explained by using the frequency response method. Finally, the modification method for the critical loop of GFM converters is analyzed by using the proposed model for system damping improvement. The electromagnetic transient simulation is performed in PSCAD/EMTDC to verify the effectiveness of the proposed model in theoretical analysis and the damping control design.