Critical sections as weak points of power system operation are the key objects to perform real-time monitoring. A weighted graph model of electrical networks is established based on the similarity evaluation of connected buses for voltage stability analysis. With the application of a recursive spectral clustering algorithm the transmission network is hierarchically partitioned into smaller sub-networks， and the critical sections are determined. The results of IEEE14-bus system and Henan power grid show that this approach can find out the critical section efficiently and accurately.