Advanced metering infrastructure is a key component of the new power system. The wide application of heterogeneous communication networks and intelligent terminals makes it vulnerable to cyber-attacks. This paper studies the dynamic modeling and optimal defending strategy of AMI network under distributed denial-of-service attacks. Firstly, the propagation path of DDoS attack in AMI network is analyzed. Combined with complex network theory and SEIR epidemic model, a state evolution model is established to describe the state evolution of nodes in AMI network after DDoS attack. The propagation mechanism and attack tolerance level of DDoS attack in AMI network are analyzed. Then, a defending strategy for flexibly optimizing the deployment of defense resources in AMI networks is proposed with the goal of minimizing defense losses and costs. Finally, the effectiveness of the proposed strategy is verified by a large number of numerical simulations under two different AMI network structures.