Information service platforms or management information systems of various institutions or sectors of enterprises are gradually interconnected to form a multi-domain environment. A multi-domain environment is convenient for managers to supervise and manage systems, and for users to access data across domains and systems. However, given the complex multi-domain environment and many users, the traditional or enhanced role-based access control (RBAC) model still faces some challenges. It is necessary to address issues such as role naming conflicts, platform–domain management conflicts, inter-domain management conflicts, and cross-domain sharing difficulties. For the above problems, a role-based access control model for inter-system cross-domain in multi-domain environment (RBAC-IC) is proposed. This paper formally defines the model, divides roles into abstract roles and specific roles, and designs the operating process of the access control model. The model has four characteristics: support role name repetition, platform–domain isolation management, inter-domain isolation management, and fine-grained cross-domain sharing. By establishing security violation formulas for security analysis, it is finally shown that RBAC-IC can operate safely.