The modular multilevel resonant DC-DC converter (MMRDC) has become a promising solution for modular multilevel DC-DC converter (MMDC), with the advantage of reducing switching losses and improving conversion efficiency. In this paper, an arm voltage balancing control method for MMRDC is studied. First, the mechanism of arm voltage imbalance in MMRDC is investigated, focusing on the impact of asymmetrical arm impedance parameters. Then a novel arm voltage balancing control method is proposed, which actively adjusts the equivalent duty cycles of the upper and lower arm modulations based on the average capacitor voltages of the submodules. The effectiveness of the proposed method is validated through simulations on the Plexim/PLECS platform and experiments on an MMRDC prototype. Results demonstrate that the proposed control method effectively eliminates arm voltage imbalance, ensuring stable operation and preventing issues such as submodule overvoltage or undervoltage and resonant capacitor overvoltage, which can lead to component damage.