Purpose: The paper aims to provide an overall methodology and case study for the innovation and extension of a digital library, especially the service system. Design/methodology/approach: Based on the three-dimensional structure theory of the information service industry, this paper combines a comprehensive analysis with the practical experiences of Peking University Library as a case study. Findings: The paper proposes three-dimensional extensions of a digital library service system. It analyses the respective functions of organisations and librarians, information resources, services and methods and the interactions between them. Originality/value: Innovatively, the theory of three-dimensional structures is applied to the discussion of the development of the digital library service system. It is significant for the integrality of a digital library theoretical system, as well as for the practical developments, innovations and sustainability of the digital library. (Contains 1 table and 3 figures.)