[Purpose/significance] The branch library has always been in the marginal position in the subject service of university libraries. In the Internet+ era, extensive subject service is becoming more sophisticated, which makes the position of a branch library changed in the subject service of university libraries. Redefining the position of a branch library will help to bring new growth points and inject vitality into the subject services of university libraries.[Method/process] Taking the subject service of branch libraries of Tsinghua University as an example, this paper introduces the principle of Position, and probes into the position and realization of a branch library in subject services of University libraries.[Result/conclusion] It is expounded that branch libraries should be regarded as the front line and frontier of the subject service.With the branch library as the core, libraries should cooperate closely with the teaching and scientific research work, and analyze and excavate the teachers and students'information requirements deeply. Based on the knowledge service and personality service, we constructs the multi-disciplinary and ubiquitous subject service mechanism of library and other related departments.