This research conducts a case study of National Science Library, affiliated to Chinese Academy of Science, to discuss the operations and characteristics of subject specialist librarian reference system in the digital era. Initiating an online subject librarian reference system since June 2006, National Science Library has integrated four branch libraries and launched a variety of innovative projects to meet the academy's research needs. To construct a complete set of knowledge service models, subject librarians have dedicated themselves to collection development, surveyed specific needs of users, and offered efficient instructions to fully interact with users. This project eventually proves to be a big success.