Hybrid modular multilevel converter (MMC) combines the advantages of half-bridge sub-module and full-bridge sub-module, and has a good application prospect. Compared with the half-bridge MMC, the charging process of hybrid MMC has changed a lot due to the addition of full-bridge sub-modules. Firstly, this paper studies and analyzes the charging characteristics of sub-modules in the case with and without transmission lines. It is found that there are some problems such as under-voltage of half-bridge sub-modules and over-voltage of full-bridge sub-modules during the uncontrolled charging period. Then, a corresponding charging strategy is proposed to prevent the above problems. A real simulation model is established in PSCAD/EMTDC. Through the simulation research in different sub-module proportion, the results verify the correctness of theoretical analysis and the feasiblity of the charging strategy.