A cluster of agricultural products supply is characterized by long and multiple chains, decentralized production, multi-source heterogeneous data, and complex network structures. A multiple chain blockchain can be widely expected in the demand for the blockchain traceability of cluster agricultural products supply chain. However, a complete traceability chain is still lacking in the multi-chain blockchain network, where the different blockchains were isolated from each other, due to the data breaks in the different alliances. In this study, an improved blockchain traceability model was proposed for the cluster of agricultural products supply chain using blockchain relay technology, in order to explore the information islands between alliances for the information connection of the entire supply chain. The business composition was also analyzed to open the upstream and downstream blockchains of the supply chain. A relay multi-chain blockchain topology was then constructed for the cluster agricultural products supply chain. The relay nodes were selected to register the related information in the application blockchain, such as cross-blockchain routing, and endorsement strategy. The blockchain was firstly initialized to interact with the destination blockchain through the relay nodes and cross-blockchain routing. A hyperledger fabric endorsement strategy was then used to verify the cross-blockchain transactions. Taking the cluster of kiwifruit supply chain as an example, the prototype system was finally designed and implemented to determine the function, performance, and scalability of the model. The integrity, credibility, and validity of cross-blockchain function were achieved to effectively solve the information chain disconnection in the multi-chain blockchain traceability. The maximum writing throughput (561.8 transactions/s) of system data was much higher than that of single chain blockchain (118.1 transactions/s), indicating the excellent performance of the model. The average data writing delay was 1.49 s, which was better than that of a single chain blockchain of 5.92 s. The number of blockchains in the system increased dynamically to flexibly respond to the changes in the cluster supply chains, according to the business needs. Consequently, the improved model can fully realize the traceability of the cluster agricultural products supply chain. The finding can provide a strong reference for the expansion of the blockchain traceability network, in order to realize the unified layout and coordinated development in the industry cluster of agricultural products supply.