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The flexible DC transmission system is connected to the island grid， so the control mode needs to be changed according to the AC topology. In the process of grid-connected and island operation conversion， there are phenomena such as excessive disturbance， high frequency instability and so on. Firstly， the transition method from grid-connected operation to island operation is put forward. The speed and accuracy of the transition from grid-connected operation to island operation of flexible DC system can be improved through the combination of electric quantity and switching quantity. Then， a simple high-frequency model of the flexible DC system is established to analyze the causes of high-frequency component during the transition from grid-connected operation to island operation. Combined with the high-frequency instability of the transition from grid-connected operation to island operation， the optimal logic of the transition from grid-connected operation to island operation and the island open-loop control strategy are proposed to improve the stability of the system. Thirdly， the transition strategy of island operation to grid-connected operation is designed based on the load forecasting of valve side current to avoid the overcurrent trip during the process from island operation to grid-connected operation. Finally， the effectiveness and correctness of the proposed strategy are verified by the online simulation model combining the control protection devices in actual engineering.