Hospitals play an important role in post-earthquake emergency response and recovery, and the assessment of post-earthquake medical functionality is important for ensuring hospital functionality after earthquakes. Such assessments of hospital departments currently face the following challenges: (1) lack of unified modeling rules and integrated platforms to manage the associated data; (2) the complex functional coupling relationships between different components and medical equipment are not often considered; (3) lack of unified communication platforms, which hinders collaboration among researchers from different fields. Therefore, this paper proposes a BIM-based method for post-earthquake medical functionality assessment (PEMFA) of hospital departments considering functional coupling. Specifically, first, multi-level of detail (LOD) BIM modeling rules for structural components, nonstructural components, and medical equipment oriented towards PEMFA are suggested. Thereafter, a conversion method for the medical functionality assessment model and PEMFA method based on BIM are proposed. Subsequently, the PEMFA method for departments is proposed, emphasizing functional coupling. Finally, a typical medical building is selected as an example to demonstrate the proposed method. The results indicate the following: (1) the fragility data for various components and equipment of medical buildings can be integrated using the proposed multi-LOD modeling rules; (2) the mapping relationship between medical functional logic and BIM has been established, through which the functional coupling relationship between different components and equipment can be fully considered, thus providing an important method for the PEMFA of hospital departments and displaying results.