Several incidents of transient power converse due to the interaction of AC/DC system have been reported recently， that led to the mal-operations of directional pilot protection in AC transmission systems. Based on CIGRE HVDC Benchmark Model， the response characteristics of HVDC transmission systems and the fault features of AC transmission systems are studied in this paper when faults occur at the inverter side AC system. The mechanism of transient power converse， the variations of AC fault features and their influence on protective relaying during the transient processes of AC system fault are analyzed. Also， some technical measures are put forward.