The interoperability of classification schemes is an urgent need for information organization and retrieval. Now, it is often to recognize and build up the mapping relationship between classes by hand in EU project Renardus and other projects. Each class of CLC or DDC can be looked as a concept, and the integrity of concept can be divided into several concept elements expressed by words. We can computer the word similarity of concept elements to reflect the similarity of the integrity of class concept on the hypothesis that the integrities are similar if all the parts of them are similar. In the automatic mapping system between CLC4 and DDC21, the concept of class can be divided into concept elements expressed by class headings, note terms, lower-class terms and upper-class terms, then rank these matched concept elements of the classes between CLC and DDC according to the principle of maximum similarity. We formulated detailed mapping regulation according to three parameters: word similarity value of concept elements, margin of concept elements and kind of equivalent concept elements of class. The article introduced the structure, functions and use of the automatic mapping system.