The Online Public Access Catalogue System of the Capital Library of China is an important way for readers to search information in the catalogue of the library. The Online Public Access Catalogue System, short as OPAC, is a way to find the information needed from documents and resource information through indexes such as title, author, publishing year, key words. There are two ways for the online information-searching, catalogue browsing and search system engine. Readers only need to submit the search request and a large amount of results will be presented from the online searching, and these results are ordered according to the relevance degree to the questions.