Abstract: Y-type modular multilevel converter (Y-MMC) is a structure that realize AC-AC conversion in high-power and high-voltage scenarios. The structure has a good prospect in the field of offshore wind power with its low manufacturing cost. When the Y-MMC is disturbed by unbalanced faults in transmission, the transmission quality is degraded. In particular, there are problems such as weak anti-interference capability, slow response, low accuracy and the hard dynamic tuning of the PI parameters when using a linear control method like the traditional PI control. In order to improve the robustness, rapidity, accuracy and adaptability of the Y-MMC under the non-ideal conditions in practical applications, this paper proposes a Lyapunov nonlinear control strategy for the low-frequency side and work-frequency side currents of the Y-MMC under the unbalanced operating conditions. Firstly, the structure of the Y-MMC circuit is analyzed and its decoupling model is derived; Then, the positive and negative sequence of the voltage and current are separated, and the mathematical expressions of positive and negative sequence Lyapunov control of the Y-MMC are derived; Finally, the simulation analysis under three unbalanced conditions is carried out on the MATLAB/Simulink simulation platform. The proposed Lyapunov control is compared with the conventional PI control, which verifies the feasibility and superiority of the proposed nonlinear Lyapunov control method for the Y-MMC control under unbalanced conditions.