Critical lines greatly promote the development of cascading failures in power system. Considering structure and operating state, a method to identify critical lines comprehensively is proposed. The critical position of lines in structure is gained by line betweenness while risk of a line's removal is obtained based on risk theory and credible theory. At last, a comprehensive index for critical lines identification is presented, which combines the perspectives of structure and operating state. Taking IEEE-RTS 24 bus system for an example, simulation results show the method effectively identifies critical lines causing large-scale blackouts.