To provide the prerequisite for protection and control strategy against cascading trips, a fast method for searching transmission section is proposed. Based on voltage phasors from wide-area measurement system (WAMS), buses of power grid for searching or regional grid are classified into two groups by Ward’s clustering method. According to the concept of maximal connected subgragh in graph theory, the method for searching generation zones and load zones within bus groups is further given. Finally, the transmission section can be identified by exploring the set of tie lines between adjacent generation zone and load zone, and the progressive method for searching transmission section and the criterion for stopping algorithm are both provided. Simulation results verify the effectiveness of the proposed method.