To study the mechanism of cascading failures in a power grid and find out the key vulnerable components propagating cascading failures， based on the basic principle of entropy and the distribution of power flow due to overload and removal of components， the components vulnerability assessment method of power flow entropy is proposed. Simulation results on the IEEE 39-bus system indicate that the smaller the distribution entropy of power flow is， the more vulnerable the node is. It also shows that the smaller the transfer entropy of power flow and the larger the impact by power flow ofgenerator-load nodes is， the more vulnerable the branch is. The vulnerable components play a key role in the propagation of cascading failures. Compared with vulnerable components evaluation results obtained by methods based on betweenness and risks， the vulnerability assessment model of power flow entropy has a clearer physical meaning， faster calculation， and is more realistic.