With the rapid development of medical information technology, the medical cyber-physical system is undergoing a rapid transformation, and the safe storage and sharing of medical data are facing great challenges. It makes the work of safe medical data storage, privacy protection and data sharing get more difficult. In this paper, we propose the combination of private blockchain and consortium blockchain that can protect information security and realize data sharing. In the system, the medical records of each node are stored in the private blockchain, and the shared data is on the consortium blockchain so as to improve the data storage and reduce data redundancy. And the threshold signature scheme without trusted center is applied in the system. In order to initiate threshold signature, a set of nodes is constructed by the sponsoring doctor, in which the threshold signature process is initiated. When there are no less than $t$ nodes sending part-signatures, the signature can be synthesized to group signature. This scheme can be well applied to the scene of multidisciplinary joint consultation in the medical blockchain. The scheme proposed in this paper has high security and computing efficiency.