When medical data terminals are used for doctor diagnosis, they can receive the corresponding medical data information in real time and transmit it to the cloud for storage after encryption. However, the keys used for data encryption and decryption are usually stored and managed directly by users or third-party organizations, resulting in the data owners losing absolute control over their data. This situation can lead to security and privacy issues, and as the number of participants increases, the time overhead also shows an unfavorable growth. To address these problems, we propose a blockchain-based medical data Shamir threshold encryption scheme. This scheme uses dynamic two-level Shamir secret sharing to split and mix the encryption private key, replacing a single long polynomial with multiple short polynomials to reduce key splitting time, and supports adaptive threshold adjustment based on the number of participants. Additionally, blockchain supervision is introduced to enhance security. We also propose an attribute-based key distribution access control scheme, constructing an “attribute relationship graph” to simplify permission management, with smart contracts automatically executing access control to ensure secure data sharing. Simulation experiments demonstrate that this scheme improves efficiency in key management, encryption and decryption, and access control, while effectively ensuring the security and privacy of shared medical data.