The article is to discuss how the theory of the life cycle of electronic documents in knowledge management system will work in public libraries. Generally speaking, A comprehensive knowledge management system must consist of an integrated document management subsystem, which provides all of explicit knowledge in print. However, a traditional public library is just a place in which contains numerous printed materials. Therefore, public libraries were supposed to be one of the best of knowledge resources. On the other hand, it is vital to understand various spectrum of each life cycle for documents to develop library computerized related services. If we realize the next step for dealing with electronic documents, we can turn public libraries into a better center for knowledge management.