If the AC lines of the convertor are removed due to various reasons, then the load shedding of AC system at convertor station side will cause the last breaker to trip, after a while a series of protective measures will be taken, such as DC ESOF. Such action increased the influence to the load area which had lost power supply from the AC lines. The system maybe facing the unstable situation when the 500kV line connected to Bao’an convertor station been cut, we utilize the 220kV lines which connected to autotransformer keep powering to the load area. We also set the power downhill parameter of Xing’an DC system and the number of reactive compensation need to remove in order to avoid the autotransformer overload. Through a series of experiments we obtained the optimum parameters which can offer basic data and technological basis for the design of practical engineering.