In this paper, we first study cloud computing technology and build a computer network security defense system based on cloud computing technology. Secondly, to better analyze the network generated by cloud computing, the corresponding network topology is established according to the characteristics of the cloud computing network, and the complete network topology is generated through four steps. Then, the avalanche effect under the cloud computing network attack and its propagation are studied. Simulated tests are used to evaluate the performance of the network security defense system security. According to the results, the test group's network security defense rate is 89.2% when the intensity of network intrusion behavior is the highest. The defense effectiveness of all eight regions in the security domain test is above 90%, which is excellent.