Abstract: With the rapid development of blockchain technology, assets such as data are no longer satisfied with circulating only on a single blockchain, so cross-chain technology has emerged. Cross-chain technology has improved the problem of blockchain data islands, but it has also brought new challenges to the transmission and sharing of data on different blockchains. On the one hand, different blockchain users usually store data in different ways, and the data verification methods of each blockchain are often not unified. Therefore, users on other blockchains may transmit false or inconsistent data, making it difficult to effectively verify the authenticity and consistency of the data, thereby affecting the reliability of cross-chain access control. On the other hand, although the current proxy re-encryption technology can effectively implement one-to-one access control, a single proxy has a centralization problem and faces the problem of high access control time overhead caused by multi-user concurrency in cross-chain scenarios. In response to the above problems, a two-stage cross-chain access control mechanism in the relay chain environment is proposed, which is designed from two stages: data verification and proxy re-encryption. In the data verification stage, to ensure the authenticity and consistency of the source of encrypted data, combined with the Fiat-Shamir heuristic and IPFS, a data verification method is designed to quickly verify the data, so that the encrypted data can be correctly received by the user, the encrypted data is securely stored in IPFS to relieve the storage pressure on the chain. In the proxy re-encryption stage, an improved threshold proxy re-encryption method is designed to effectively deal with access control in multi-user scenarios, and the re-encryption process is optimized based on the re-encryption update algorithm. While achieving decentralized re-encryption, the time overhead of re-encryption is reduced, and the efficiency of re-encryption transaction processing for multi-user access control is effectively improved. Experimental results show that the proposed mechanism has a small overhead in data verification, and the overhead of encryption, re-encryption, and decryption processes in multi-user scenarios is better than the comparison method.