Abstract: The flexible DC grid is an important technical measure for large-scale long-distance overhead line transmission of renewable energy. However, overhead lines are more prone to instantaneous short-circuit faults than DC cables, and they need to be reclosed quickly after the fault is cleaned. If a permanent fault occurs in the DC grid, the conventional fault identification method based on the active signal injection of the converter is likely to cause great interference to the stable operation of the DC grid. To solve these problems, an improved adaptive reclosing strategy with active signal injection of the hybrid modular multilevel converter（MMC） for flexible DC grids is presented. Active signal control is added to the hybrid MMC pole controller, so that the flexible DC grid can realize the identification of the fault properties without interrupting the power transmission. This method has strong resistance to transition resistance and does not affect the power transmission stability of the flexible DC grid. The electromagnetic transient model of the four-terminal flexible DC grid with hybrid MMC is built in PSCAD/EMTDC. The simulations verify the effectiveness of the method in the flexible DC grid equipped with hybrid MMC and mechanical DC circuit breakers.