To study how large-scale blackouts take place and find out the key factors affecting the spread of cascading failures， based on the entropy theory， a new concept of power flow entropy is proposed to quantify the unevenness extent of the power flow， and an identification model for self-organized criticality of power grids is proposed based on the power flow entropy. Simulation results on the IEEE 118-bus system indicate that the power flow entropy has important influence on the spread of cascading failures， and is one of the key factors determining the power grid whether to evolve into its self-organized criticality. The value of power flow entropy of the IEEE 118-bus system is presented when the system reaches its self-organized criticalities at given load levels. The power flow entropy reasonably elucidates why practical power systems with low average load level can reach their self-organized criticalities.