Abstract: To cope with the problem of high complexity in extraction of functional regularity in digital ICs,an algorithm capable of categorizing the root nodes gradually is proposed.By extending the frequent edges directly,the small frequent subcircuits can be extracted fast;and utilizing structure dependencies between small frequent subcircuits and big ones at gate level,the combination explosion problem of root nodes has been solved in the process of generating candidate subcircuits.Experimental results show that the proposed algorithm can reduce the number of root nodes effectively,extract the high frequency candidate subcircuits with high priority and reduce runtime of regularity extraction observably.