The wide application of the Internet and the use of various network devices make the existing network scale increasingly complex, and the hidden dangers of various network security incidents are gradually exposed. Software Defined Network (SDN) brings potential security risks and challenges while changing the underlying network structure. In this paper, a situation assessment model based on SDN network is proposed by summarizing the important indicators and parameters of SDN. Then, the situational factors are mapped to the layers of the neural network, and the weights and thresholds of the back propagation(BP) neural network are optimized by the improved algorithm to obtain the global optimal solution. Finally, the purpose of situation assessment and comprehensive rating of the SDN network is realized. The experimental results show that the situation assessment method proposed in this paper has good effect and high accuracy, which provides a better basis for the next situation prediction.