The number of modular multilevel converter (MMC) sub-modules (SMs) is proportional to the voltage of DC side. When the SM number increases, the switching loss of MMC will increase sharp, so reducing the switching frequency of power devices has been an important research direction of MMC. The nearest level modulation (NLM) is adopted and an improved equal-voltage ranking method based on a full-bridge SM is proposed, aiming to reduce the switching frequency of IGBT in MMC. This method is relatively simple, requires no additional controller and is easy to be extended. Then, 19 simulation models of full-bridge SM are built on MATLAB/Simulink platform to verify the effectiveness of the method. It is verified that the proposed full-bridge SM optimization voltage-sharing strategy can effectively avoid unnecessary repeated switching of IGBT, reduce the switching loss of IGBT, and have no negative effects on the external output characteristics.