For the most widely used Office Open XML (OOX) format documents (MS Office 2007—2013),this paper proposes a robust watermarking algorithm based on the redundant attribute.Watermarking information,which is linked to the format information of document,can be imperceptibly embedded into the redundancy space of OOX parts.The method can resist text content-based active attacks.The method this paper proposed has better robustness and embedding capacity,which can resist "Clean Format","Save As" and other active attacks.The proposed watermarking method does not change any format or semantic information of the original documents,which no existing methods would do.The experiment demonstrates the feasibility of the algorithm.