Due to the lack of generators at sending end in the early operation stage of the Dianxibei UHVDC project, if it is passively entered into island mode, the system will encounter harmonic instability and will cause a great risk to the steady operation. In previous projects, the strategy is to disconnect AC filters and block bipolar DC with a certain time delay when receiving island operation signal. However HVDC system may block valves, cut generation capacity or shed power by error when receiving false island operation signal. Therefore, the locking strategy at low power under passive island mode is optimized. In Dianxibei UHVDC project, the power on AC tie lines is used as criteria of island operation to prevent from miscarriage. The logic and realization of the control strategy are illustrated, then RTDS simulations are carried out and it is verified that by judging the power on AC tie lines, the system can effectively prevent entering island mode by error. Finally, the field test proved that the proposed optimized strategy will not cause rejecting of the existing “island locking logic”. The proposed strategy can be a reference to other DC projects.