The library of Tsinghua University was originally built in 1912.For the past century of its history,the library has been expanded and renovated for 3 times (first stage : completed in 1919,second stage: completed in1931;third stage: completed in 1991),and the fourth stage is currently under construction.Built in different time,the library varies significantly in its indoor function and space arrangement,despite its unified and integrating building styles looking from the outside,and in each building block a specific pattern of environmental behavior related to its unique architectural space is thus developed,which as a whole outline a vivid picture of the history the interactions between library users and the physical environment.By surveying both the librarians and the readers of their personal experience and individual behavior,this paper analyses how such variety of behavior patterns are formed and how they relate to the architectural space built in different era.From a perspective of architectural programming and environmental behavior,the paper discussed some interesting outcomes which might be useful to library design today.