Blockchain technology has greatly promoted the development of the Internet of Medical Things (IoMT) from a centralized form to a distributed form. However, with many independent blockchain-based IoMT (BIoMT) systems being established by different institutions, these heterogeneous blockchain systems have created a new "data islands" problem. This paper proposes a multi-chain fusion (MCF) multi-party governance model based on relay chain technology. The MCF model helps establish cross-chain interaction and communication among the multiple heterogeneous blockchain systems in different medical institutions through the cross-chain gateway. Meanwhile, by utilizing the inter-blockchain transfer protocol (IBTP), the MCF model takes responsibility for legitimacy verification and reliable routing of cross-chain transactions, thus addressing the "data islands problem" inherent in traditional blockchain-based medical systems. Then, an elliptic curve signature scheme is adopted to protect user privacy in cross-chain transactions, which helps establish transaction verification for the MCF model in BIoMT systems. Finally, several experiments are carried out to prove the feasibility of the cross-chain scheme and the security of the elliptic curve signature scheme, and the performance index of the cross-chain system is evaluated. The performance results show that the MCF model can provide secure and efficient cross-chain services for sharing medical data among heterogeneous BIoMT systems, thus addressing the "data islands problem" inherent in traditional blockchain-based medical systems. This paper proposes a multi-chain fusion (MCF) multi-party governance model based on relay chain technology. The MCF model helps establish cross-chain interaction and communication among the multiple heterogeneous blockchain systems in different medical institutions through the cross-chain gateway. Meanwhile, by utilizing the inter-blockchain transfer protocol (IBTP), the MCF model takes responsibility for legitimacy verification and reliable routing of cross-chain transactions. Then, an elliptic curve signature scheme is adopted to protect user privacy in cross-chain transactions, which helps establish transaction verification for the MCF model in BIoMT systems. image