Abstract: Currently, most hospitals have adopted electronic medical record (EMR), making the secure and controllable sharing of EMRs an urgent problem to be addressed. Traditional solutions primarily employ public key encryption to protect patients' private data, requiring the distribution of public key certificates to each patient. However, in medical scenarios involving a large number of patients, the storage and verification costs of patient public key certificates can be significantly high. To tackle this issue, we propose a novel EMR sharing scheme that integrates blockchain technology and the SM9 identity-based cryptosystem. By conducting an in-depth analysis of the SM9 encryption algorithm, we improve upon it by introducing a proxy re-encryption mechanism and formally proving the security of the enhanced algorithm. In the proposed scheme, patients' encrypted medical records are stored on the blockchain. When a patient visits a different hospital, they can quickly retrieve the ciphertext of their medical records and securely share the re-encrypted ciphertext with doctors through smart contracts, enabling secure access to the records. Experimental results demonstrate that the improved algorithm can complete re-encryption operations within milliseconds. The proposed scheme not only reduces the management overhead of public key certificates but also ensures the data security of electronic medical records.