In allusion to the current situation of network security assessment which is hard to be quantified,a network security situation assessment model based on NetFlow is proposed after considering many factors that affect network security.Proceeding from the attacks to network services,the hierarchical assessment strategy was adopted,and the risk value of each service,the host security index and defense index were obtained by model through the quantization calculation of different threatening elements.Meanwhile the problem mentioned above was solved and the evaluation of the security situation for the entire network was realized.Two comparative results are offered according to a simulation experiment.The results show that the method to consider the defense index can better reflect the network security situation and assist the security administrator to make correct decisions.