A 100 Hz DC protection may mal-operate due to the fault in AC system even if the fault location is very far from the converter station. Unfortunately， there is no effective method to indicate fault location range in AC system that may cause a 100 Hz protection operate. A practical method， which combines the electric-magnetic transient simulation with the short-circuit calculation， is proposed. Using this method， the 500 kV network of 2007 China Southern Power Grid is studied， and the range of fault location in AC system that influences the characteristic of a 100 Hz protection is indicated with a number of fault forms. The analysis for Case Yunnan“6.23” verifies the correctness and validity of the proposed method.