In recent years, with the rapid development of blockchain, blockchain technology has been widely used in various scenarios. The application of blockchain in the carbon trading market is of great significance to help achieve carbon peak and carbon neutrality goals. However, the performance of the traditional blockchain cannot meet the needs of large-scale and high-concurrent transactions in the carbon market. At the same time, the blockchain is also facing huge storage pressure in the face of large-scale carbon data. To solve the problem of on-chain storage pressure, we propose a carbon trading system architecture based on on-chain and off-chain collaborative storage, which standardizes the execution steps of carbon trading and the process of data storage and acquisition. To improve the performance of blockchain, we propose a double layer consensus optimization mechanism in DAG(Directed Acyclic Graph)-based blockchain. The local sharding and global sharding form a two-layer consensus architecture, and the Sim-Hashgraph consensus algorithm is proposed to improve the consensus efficiency. Finally, experimental simulations verify that the Sim-Hashgraph consensus algorithm can effectively improve the consensus efficiency, and verify the effectiveness of the Sim-Hashgraph based sharding blockchain architecture.