The problem of harmonic distortion in the AC-side output current and DC-side circulating current of the modular multilevel converter (MMC) under unbalanced grid voltage conditions is addressed by proposing a control strategy based on the red-tailed hawk algorithm (RTH) optimized sliding mode. Firstly, the unbalanced current is decomposed according to the MMC′s topology, and its mathematical model under unbalanced grid voltage is established. Then, a novel reaching law is introduced, its performance is analyzed, and its stability is verified through the Lyapunov function. On this basis, the red-tailed hawk optimization algorithm is applied to dynamically optimize the sliding mode control of the new reaching law. Finally, through simulation experiments on the MATLAB/Simulink platform, comparisons are made with integral sliding mode control and the new reaching law sliding mode control without optimization. The results show that the proposed method has significant advantages in both dynamic response and steady-state accuracy.