C# 可以使用OpenXML ：
Inserting Watermark Using OpenXML SDK

using DocumentFormat.OpenXml.Packaging;
using DocumentFormat.OpenXml.Wordprocessing;
using DocumentFormat.OpenXml;
using V = DocumentFormat.OpenXml.Vml;

 using (WordprocessingDocument package = WordprocessingDocument.Open(@"C:\Users\*****\****.docx", true))
            {
                InsertCustomWatermark(package, @"C:\Users\*******\****.jpg");
            }


   private void InsertCustomWatermark(WordprocessingDocument package, string p)
        {
            SetWaterMarkPicture(p);
            MainDocumentPart mainDocumentPart1 = package.MainDocumentPart;
            if (mainDocumentPart1 != null)
            {
                mainDocumentPart1.DeleteParts(mainDocumentPart1.HeaderParts);
                HeaderPart headPart1 = mainDocumentPart1.AddNewPart<HeaderPart>();
                GenerateHeaderPart1Content(headPart1);
                string rId = mainDocumentPart1.GetIdOfPart(headPart1);
                ImagePart image = headPart1.AddNewPart<ImagePart>("image/jpeg", "rId999");
                GenerateImagePart1Content(image);
                IEnumerable<SectionProperties> sectPrs = mainDocumentPart1.Document.Body.Elements<SectionProperties>();
                foreach (var sectPr in sectPrs)
                {
                    sectPr.RemoveAllChildren<HeaderReference>();
                    sectPr.PrependChild<HeaderReference>(new HeaderReference() { Id = rId });
                }
            }
            else
            {
                MessageBox.Show("alert");
            }
        }
        private void GenerateHeaderPart1Content(HeaderPart headerPart1)
        {
            Header header1 = new Header();
            Paragraph paragraph2 = new Paragraph();
            Run run1 = new Run();
            Picture picture1 = new Picture();
            V.Shape shape1 = new V.Shape() { Id = "WordPictureWatermark75517470", Style = "position:absolute;left:0;text-align:left;margin-left:0;margin-top:0;width:415.2pt;height:456.15pt;z-index:-251656192;mso-position-horizontal:center;mso-position-horizontal-relative:margin;mso-position-vertical:center;mso-position-vertical-relative:margin", OptionalString = "_x0000_s2051", AllowInCell = false, Type = "#_x0000_t75" };
            V.ImageData imageData1 = new V.ImageData() { Gain = "19661f", BlackLevel = "22938f", Title = "水印", RelationshipId = "rId999" };
            shape1.Append(imageData1);
            picture1.Append(shape1);
            run1.Append(picture1);
            paragraph2.Append(run1);
            header1.Append(paragraph2);
            headerPart1.Header = header1;
        }
        private void GenerateImagePart1Content(ImagePart imagePart1)
        {
            System.IO.Stream data = GetBinaryDataStream(imagePart1Data);
            imagePart1.FeedData(data);
            data.Close();
        }
        private string imagePart1Data = "";
        private System.IO.Stream GetBinaryDataStream(string base64String)
        {
            return new System.IO.MemoryStream(System.Convert.FromBase64String(base64String));
        }
        public void SetWaterMarkPicture(string file)
        {
            FileStream inFile;
            byte[] byteArray;
            try
            {
                inFile = new FileStream(file, FileMode.Open, FileAccess.Read);
                byteArray = new byte[inFile.Length];
                long byteRead = inFile.Read(byteArray, 0, (int)inFile.Length);
                inFile.Close();
                imagePart1Data = Convert.ToBase64String(byteArray, 0, byteArray.Length);
            }
            catch (Exception ex)
            {
                Debug.Print(ex.Message);
            }
        }