Abstract: Watermark technique is one of effective means of copyright protection.On the basis of a detailed analysis of natural language watermarking,a text watermarking algorithm based on sentence length is proposed.The watermarking,with the modification of sentence length by sentence transformations,can be embedded into the document.The sequence of paragraphs and sentences is permuted using DES and hashing table in order to improve the watermarking robustness before watermarking embedding,and during the embedding,redundant embedding and majority voting are introduced.Analysis and provement show that the watermarked documents have good robustness and the false positives is extremely small.