Power system blackout after power flow transfers caused by overloaded line tripping often occurs. To avoid cascading tripping， real-time emergency control is necessary. The key issue is to predict whether there are new overloaded lines in the system effectively and accurately. The paper firstly forms the system's minimal basic circuit set using graph theory on-line based on network topology analysis， together with component parameters and system operation state. Secondly， after a line is overloaded， it obtains circuit containing the overloaded line， searches for the lines which have the opposite power flow direction to the overloaded line in the circuit， and extends the search range using ring sum operation. Finally， it calculates in series power flow of a line after the overloaded line trips using distribution factors to confirm the lines to whom most of the power flow of the tripped line has been transferred. As a result， the computational needs for further analysis and system security assessment can be significantly reduced.