Component importance is the key parameter of reliability analysis in power converter. Because of the particularity of topology, the traditional importance evaluation methods are not completely suitable for power converter. Based on complex network theories, the topology of power converter can be transformed to a complex network, and a method to evaluate component importance is proposed by using importance evaluation matrix, in which we synthesize node closeness, node degree and importance contributions of adjacent node. And the time complexity of this method is lower than O(n3). Finally, the experience of Buck converter verified this method is effective and feasible, and results are in conformity with actual fact.