Fast search of the key transmission sections plays an important role in the protection and control of power system. A fast search method of the key transmission sections based on clustering algorithms is proposed. The weight of lines is given considering both line betweenness and power flow. The similarity matrix can be obtained based on the shortest distance of two buses which is got through Dijkstra algorithms. Then Affinity Propagation (AP) algorithm is used to construct the partition of power grid. Based on the partition, combined with the constraints of spanning tree proposed, fast search of the transmission sections can be completed through the basic cut-sets searching theoty. Finally, key sections can be sorted according to the safety margin of the section. The analysis results of New England 39-bus standard system verify the validity of the method.