High-voltage direct current transmission has a high transmission reliability, but when the high voltage grade ac lines connected to the inverter side out of service, we can choose not to blocking dc system, instead of using the low voltage grade lines which connected to the inverter side continue transmission power. However, this situation may cause inverter side facing the danger of overload, at the same time it may reducing the stability of the system. The HVDC system may caused a serious impact when the DC power decreased rapidly, and the HVDC system could not increase the system stability when the DC power decreased slowly. So it’s very important that choose a appropriate DC power decrease rate. In this paper, we adopted the way of dropping the DC operating power to avoid the possibility of happen the overload, analyzed the mechanism of choosing the appropriate dc power reduction rate to increase the stability of the system which based on extended equal area method under fault condition.