The occurrence of the high voltage direct current (HVDC) system fault often results in adverse effect on the stability of nearby connected ac grid. To avoid such effect, many HVDC transmission projects with sending terminal mainly composed of one or more giant hydropower stations chose islanding operation mode as one of its normal operation modes. However, in that case, once bipolar blocking caused by HVDC system fault occurs, the amount of power transmission will drop dramatically and two problems of the islanding ac system will emerge: 1) some of the units will stabilize into motor mode; 2) system stable frequency will become much higher than the normal value. To solve these problems, combining with practical engineering, reasons of them are analyzed first, then supplementary control strategy of hydro-turbine governor is proposed in this paper. Results of the simulation and field test showed that the strategy proposed in this paper can effectively avoid the occurrence of the two problems. Consequently, the supplementary control strategy is beneficial to the fault recovery and restarting of the HVDC system, and can also enhance the safety running level of the hydro-turbine units and grid connected.