In order to improve the stability and signal tracking ability of existing flexible grounding devices to suppress the threephase unbalance in distribution networks, a flexible grounding control method is proposed based on the improved double closedloop control strategy. Firstly, the control system is modeled and analyzed, and an injection current control strategy is proposed. This strategy is based on the double closedloop control and it adopts the current innerloop PI control and voltage outerloop hysteresis network control. By setting the reference value of neutral point voltage to zero and taking it as the control target, the difference between the reference value and the feedback value of neutral point voltage is compared in real time. And then the difference goes through the series lag correction. Thus, it can be treated as a reference current in the current inner loop. The deviation between the theoretical reference current and the actual measurement current is taken as the input of PI correction link. For the outer loop output, the target control for the output current is realized, the stability margin of current is effectively improved and the tracking time of output and input signals is shorten. The good performance of proposed method in threephase unbalanced overvoltage suppression is verified by a system simulation analysis. Neutral point voltage in distribution network is accurately suppressed to zero.