The sharing of medical data among medical institutions plays an important role in realizing cross-hospital diagnosis and promoting the development of medical research. In order to solve difficult medical data sharing among medical institutions, this study proposes a cross-chain-based medical data security sharing scheme. The scheme adopts a cross-chain architecture combining relay chains, cross-chain gateways, and data chains and conducts fine-grained medical data access control based on AES and CP-ABE encryption algorithms. Moreover, searchable encryption is used to realize the safe search of medical data. In order to alleviate the computational storage overhead of blockchain, IPFS and the blockchain are combined for off-chain storage of ciphertext and on-chain storage of ciphertext addresses and keys. The security analysis and experiment prove that the scheme is feasible in medical data security sharing.