Knowledge service is a kind of value-added services. It bases on all information resources and aims at needs of users. The research on the library information ecosystem can solve the problem about the disorder of library information ecological environment and the information ecological unbalance of modern library service. According to the continuously large contradictions between knowledge service and unbalanced information, this research analyzes the basic principles of library knowledge service based on the information ecology, including the principle of information ecological balance and the principle of the knowledge service. The key component of this essay is the research of both of them, which are the theoretical basis of systematical construction and management of library knowledge service based on information ecology. In the end, this research establishes the knowledge service platform based on the analysis above including resource database of information, business platform of librarian, knowledge base, and service platform of user. And four management measures are proposed that are the building usercentered evaluation system of ecology, the realization of ecological management of knowledge, the improvement of the main quality of information ecology and the establishment of the ecosphere of library information.