Abstract: The fast and reliable online connection of modular multilevel converter (MMC) to DC grid is one of the problems needing to be solved urgently in practice. Since the islanded converter station cannot be started up by AC grid, it is necessary to start up the converter by the DC grid and realize its connection to the grid. In order to limit the charging current at the DC side, the DC breaker can be used to pre-charge the converter. The feasibility is studied to use step by step switching on of DC breaker to realize the online connection of islanded converter station. An innovative calculation method is also proposed to calculate the current stress of DC breaker and the MOV energy in the process of step by step switching on, and is applied to the calculation of the electrical stress of DC breaker in Zhangbei DC project. The Zhangbei flexible DC grid project is modelled and simulated based on PSCAD/EMTDC, and the results validate the applicability in DC grid of the proposed online DC grid connection method and the correctness of the proposed calculation method.