Continuous commutation failure of DC transmission system may cause transient instability in AC/DC interconnected power system. The main reason for transient instability is that continuous sudden-reduction of DC power under continuous commutation failure condition results in generator’s cumulative acceleration area greater than deceleration area. The emergency control scheme for coping with the transient instability problem by security and stability control devices is proposed. The accumulative commutation failure time is adopted as instability criterion and matching variable of the generator tripping scheme. If continuous commutation failure whose duration is less than the blocking threshold value, the generator-tripping measures are adopted to prevent the instability, otherwise, the DC system blocking and generator-tripping measures are adopted simultaneously to ensure the stability. The effectiveness of this method is verified by the case of actual AC/DC interconnected power grid.