Network architecture is key to mobile communication networks, which determines network resource efficiency, service modalities, and industry trends. In the future 6G network, the ever-changing scenarios, refined service classification, and diversified network elements will make the design and deployment of network architecture more difficult and complex. Further, digital twin, as a real-time interactive mapping network system, can create a mirror image of physical space in the digital world and provide strong support for the mathematical modeling of network architecture. Therefore, this paper proposes a method to model, evaluate, and optimize 6G network architecture based on digital twin. First, the 6G network mapped into the digital twin is divided into service, logical, and resource domains. Then, the 6G network architecture is mathematically modeled by intra-domain and inter-domain hypergraphs to characterize the complex changing rules of various elements in the network architecture and their relationships with the supply of services. Next, to effectively evaluate the rationality of the design and deployment of the 6G network architecture, the network architecture entropy is defined from the intra- and inter-domain dimensions. Finally, two specific simulation case studies for minimizing the overall network architecture entropy are analyzed based on hypergraphs and network architecture entropy, showing the viability of the proposed architecture modeling method and offering specific suggestions for optimal 6G network design and deployment.