In recent years, electronic medical records(EMRs) sharing has played a vital role in formulating optimized treatment plans, providing data sets for researchers, and accelerating the development of biomedical science. However, This unprecedented era of technological confluence poses significant data security and privacy challenges. To solve these problems, we propose a blockchain-based multi-keyword search scheme for medical data sharing called BMKS, focusing on ensuring medical data confidentiality and realizing secure data retrieval. In BMKS, we introduce a two-level search scheme to achieve efficient and verifiable keyword searches. The introduction of the improved Bloom filter significantly improves query efficiency. In addition, we take advantage of blockchain technology to realize ciphertext search and pre-decryption, which reduces user's decryption overhead. Moreover, blockchain's transparency and tamper-preventing characteristics help record the access control process in a traceable and auditable way. The performance evaluation and security analysis show that BMKS is comprehensively safe, efficient, and practical.