Decarbonization is a trend in the future development of new power systems. At the planning level, simulating the annual carbon trading volume plays a significant role in achieving a low-carbon economy in the future power grid operation. Considering the high interconnectedness of the grid, this paper proposes a decentralized transmission network planning model that takes carbon trading into account. Firstly, a carbon trading mechanism model based on the benchmark method is established. Then, following the idea of the Analytical Target Cascading algorithm, the planning model is decoupled into independent sub-models using the interconnection power of the tie lines as coupling variables. Finally, the improved 48-bus system is simulated to verify the proposed model. The results indicate that decentralized optimization can achieve comparable performance to centralized optimization within an acceptable margin of error. Additionally, it enables synergistic optimization of both low-carbon and economic aspects.