The invention relates to a reliability value increment based method and system for identifying critical lines. According to the method and the system, on the basis of the consideration of electric power system reliability influenced by line cutoff and line capacity limitation, probability statistics is used for calculating average reliability increment brought from the line cutoff and line capacity limitation, the critical lines are searched through the ordering of total average reliability increments, and meanwhile, cross entropy method sampling simulation is used for accelerating the calculation process. The method and the system are quick in calculation, the identified critical lines are high in reliability, and a reliable and powerful decision basis can be provided for electric power system planning evaluation.