Purpose/Significance To explore the ways for hospital libraries to provide subject services based on mapping knowledge domains. Method/Process The paper summarizes subject services implemented by hospital libraries based on mapping knowledge domains from aspects of reference consulting service, information attainment education service, discipline resource construction service, scientific research support service, management and decision-making support service. It elaborates on concrete practice of the library of The Fourth Hospital of Hebei Medical University based on mapping knowledge domains, including improvement of professional attainment of discipline librarians, information attainment education of readers, and scientific research support service. Result/Conclusion Hospital librarians should improve their ability to interpret the mapping knowledge domains, and comprehensively use the visual analysis function of database documents and artificial intelligence scientific research assistance tools to ensure high-quality development of subject services.