Document copy detection is such a behaviour which judge whether a document is cribbed from another or some other documents. There are many algorithms in this domain. The algorithms based on the similarity of the sentences is a good one, which not only emphasizes on the whole document, but also pays attention to the structure of the document. In the paper, the authors improve the similarity algorithm based on it， and provide a new algorithm which aims to check the Chinese documents. Our algorithm use sentence as the basic item of a document, make some improvement to the old methods. The algorithm solves the artificial problem of threshold setting and improves the detection accuracy, and the result of experiments shows that it is feasible.