In response to the problems of the control method used in modular multilevel converter (MMC)-unified power flow controller (UPFC), a nonlinear control strategy combining back stepping control (BSC) and sliding mode control (SMC) based on MMC-UPFC is proposed in this paper. This control strategy not only retains the strong antiinterference ability of SMC, but also has the advantages of fast response speed of BSC. Firstly, according to the topology of MMC-UPFC, the mathematical models for the parallel-side and the seriesside of MMC converter is established. Secondly, positive and negative sequence decomposition is carried out under unbalanced conditions, and the BSC-SMC controller for MMC-UPFC under unbalanced conditions is designed, and the stability of the control system is proved. Finally, a MMC-UPFC simulation model is established on the MATLAB/Simulink software platform for simulation verification, by comparing the BSC-SMC control strategy with PID control strategy under two different operating conditions: Single-phase drop of power supply voltage at the receiving end and injection voltage on the series-side of MMC, the proposed BSC-SMC control strategy not only has the strong anti-interference characteristics of SMC control strategy, but also has the characteristics of fast response speed and small overshoot of BSC control strategy, the stability and robustness of the control system are stronger, the effectiveness and superiority of BSC-SMC control strategy is verified.