using System;
using System.Drawing;
using System.Windows.Forms;
using Spire.Xls; 

private static System.Drawing.Image DrawText(String text, System.Drawing.Font font, Color textColor, Color backColor, double height, double width) <br>{
 //创建一个指定宽度和高度的位图图像
 Image img = new Bitmap((int)width, (int)height);
 Graphics drawing = Graphics.FromImage(img);
 //获取文本大小
 SizeF textSize = drawing.MeasureString(text, font);
 //旋转图片
 drawing.TranslateTransform(((int)width - textSize.Width) / 2, ((int)height - textSize.Height) / 2);
 drawing.RotateTransform(-45);
 drawing.TranslateTransform(-((int)width - textSize.Width) / 2, -((int)height - textSize.Height) / 2);
 //绘制背景
 drawing.Clear(backColor);
 //创建文本刷
 Brush textBrush = new SolidBrush(textColor);
 drawing.DrawString(text, font, textBrush, ((int)width - textSize.Width) / 2, ((int)height - textSize.Height) / 2);
 drawing.Save();
 return img;
}

Workbook workbook = new Workbook();
workbook.LoadFromFile(@"C:\Users\Administrator\Desktop\sample.xlsx");

Font font = new System.Drawing.Font("arial", 40);
String watermark = "内部资料";
foreach (Worksheet sheet in workbook.Worksheets)
{
 //调用DrawText()方法新建图片
 System.Drawing.Image imgWtrmrk = DrawText(watermark, font, System.Drawing.Color.LightCoral, System.Drawing.Color.White, sheet.PageSetup.PageHeight, sheet.PageSetup.PageWidth);
 //将页眉图片设置为左对齐
 sheet.PageSetup.LeftHeaderImage = imgWtrmrk;
 sheet.PageSetup.LeftHeader = "&G";
 //水印只会在此种模式下显现
 sheet.ViewMode = ViewMode.Layout;
 }

workbook.SaveToFile("水印.xlsx", ExcelVersion.Version2010);
System.Diagnostics.Process.Start("水印.xlsx");

using System;
using System.Drawing;
using System.Windows.Forms;
using Spire.Xls;
 
namespace Add_Watermark_To_Excel
{
 public partial class Form1 : Form
 {
  public Form1()
  {
   InitializeComponent();
  }
 
  private void button1_Click(object sender, EventArgs e)
  {
   //初始化一个新工作簿并加载要添加水印的文件
   Workbook workbook = new Workbook();
   workbook.LoadFromFile(@"C:\Users\Administrator\Desktop\sample.xlsx");
   //在页眉插入图片
   Font font = new System.Drawing.Font("arial", 40);
   String watermark = "内部资料";
   foreach (Worksheet sheet in workbook.Worksheets)
   {
    //调用DrawText()方法新建图片
    System.Drawing.Image imgWtrmrk = DrawText(watermark, font, System.Drawing.Color.LightCoral, System.Drawing.Color.White, sheet.PageSetup.PageHeight, sheet.PageSetup.PageWidth);
    //将页眉图片设置为左对齐
    sheet.PageSetup.LeftHeaderImage = imgWtrmrk;
    sheet.PageSetup.LeftHeader = "&G";
    //水印只会在此种模式下显现
    sheet.ViewMode = ViewMode.Layout;
   }
   workbook.SaveToFile("水印.xlsx", ExcelVersion.Version2010);
   System.Diagnostics.Process.Start("水印.xlsx");
  }
  <br>  private static System.Drawing.Image DrawText(String text, System.Drawing.Font font, Color textColor, Color backColor, double height, double width)
  {
   //创建一个指定宽度和高度的位图图像
   Image img = new Bitmap((int)width, (int)height);
   Graphics drawing = Graphics.FromImage(img);
   //获取文本大小
   SizeF textSize = drawing.MeasureString(text, font);
   //旋转图片
   drawing.TranslateTransform(((int)width - textSize.Width) / 2, ((int)height - textSize.Height) / 2);
   drawing.RotateTransform(-45);
   drawing.TranslateTransform(-((int)width - textSize.Width) / 2, -((int)height - textSize.Height) / 2);
   //绘制背景
   drawing.Clear(backColor);
   //创建文本刷
   Brush textBrush = new SolidBrush(textColor);
   drawing.DrawString(text, font, textBrush, ((int)width - textSize.Width) / 2, ((int)height - textSize.Height) / 2);
   drawing.Save();
   return img;
  }
 
 } 
}