PM> Install-Package Spire.XLS

using Spire.Xls;
using System.Drawing;

namespace AddWatermark
{
    class Program
    {
        static void Main(string[] args)
        {
            //初始化 Workbook 类的实例并加载 Excel 文件
            Workbook workbook = new Workbook();
            workbook.LoadFromFile("示例文档.xlsx");

            //创建字体
            Font font = new Font("宋体", 40);
            //定义文本
            string watermark = "内部使用";

            //遍历文件中的所有工作表
            foreach (Worksheet sheet in workbook.Worksheets)
            {
                //调用 DrawWatermarkImage() 方法根据文本创建图像
                Image imgWtrmrk = DrawWatermarkImage(watermark, font, Color.LightCoral, Color.White, sheet.PageSetup.PageHeight, sheet.PageSetup.PageWidth);

                //将图像添加到每个工作表的左侧标题部分
                sheet.PageSetup.LeftHeaderImage = imgWtrmrk;
                sheet.PageSetup.LeftHeader = "&G";

                //将工作表的查看模式更改为页面布局以查看水印
                sheet.ViewMode = ViewMode.Layout;
            }

            //保存结果文件
            workbook.SaveToFile("添加水印.xlsx", ExcelVersion.Version2013);
        }

        private static Image DrawWatermarkImage(string text, Font font, Color textColor, Color backColor, double height, double width)
        {
            //创建具有指定宽度和高度的图像
            Image img = new Bitmap((int)width, (int)height);
            //从图像创建一个 Graphics 对象
            Graphics drawing = Graphics.FromImage(img);

            //获取文字的大小
            SizeF textSize = drawing.MeasureString(text, font);

            //通过将指定的转换前置到图形的变换矩阵来更改坐标系统的原点
            drawing.TranslateTransform(((int)width - textSize.Width) / 2, ((int)height - textSize.Height) / 2);

            //应用旋转
            drawing.RotateTransform(-45);

            //通过将指定的转换前置到图形的变换矩阵来更改坐标系统的原点
            drawing.TranslateTransform(-((int)width - textSize.Width) / 2, -((int)height - textSize.Height) / 2);

            //绘制背景
            drawing.Clear(backColor);

            //为文本创建画笔
            Brush textBrush = new SolidBrush(textColor);

            //将文本绘制到图形的中心位置
            drawing.DrawString(text, font, textBrush, ((int)width - textSize.Width) / 2, ((int)height - textSize.Height) / 2);
            drawing.Save();
            return img;
        }
    }
}

Imports Spire.Xls
Imports System.Drawing

Namespace AddWatermark
    Friend Class Program
        Private Shared Sub Main(ByVal args As String())
            '初始化 Workbook 类的新实例并加载 Excel 文件
            Dim workbook As Workbook = New Workbook()
            workbook.LoadFromFile("示例文档.xlsx")

            '创建字体
            Dim font As Font = New Font("宋体", 40)
            '定义文本
            Dim watermark = "内部使用"

            '遍历文件中的所有工作表
            For Each sheet As Worksheet In workbook.Worksheets
                '调用 DrawWatermarkImage() 方法根据文本创建图像
                Dim imgWtrmrk As Image = DrawWatermarkImage(watermark, font, Color.LightCoral, Color.White, sheet.PageSetup.PageHeight, sheet.PageSetup.PageWidth)

                '将图像添加到每个工作表的左侧标题部分
                sheet.PageSetup.LeftHeaderImage = imgWtrmrk
                sheet.PageSetup.LeftHeader = "&G"

                '将工作表的查看模式更改为页面布局以查看水印
                sheet.ViewMode = ViewMode.Layout
            Next

            '保存结果文件
            workbook.SaveToFile("添加水印.xlsx", ExcelVersion.Version2013)
        End Sub

        Private Shared Function DrawWatermarkImage(ByVal text As String, ByVal font As Font, ByVal textColor As Color, ByVal backColor As Color, ByVal height As Double, ByVal width As Double) As Image
            '创建具有指定宽度和高度的图像
            Dim img As Image = New Bitmap(width, height)
            '从图像创建一个 Graphics 对象
            Dim drawing As Graphics = Graphics.FromImage(img)

            '获取文字的大小
            Dim textSize As SizeF = drawing.MeasureString(text, font)

            '通过将指定的转换前置到图形的变换矩阵来更改坐标系统的原点
            drawing.TranslateTransform((CInt(width) - textSize.Width) / 2, (CInt(height) - textSize.Height) / 2)

            '应用旋转
            drawing.RotateTransform(-45)

            '通过将指定的转换前置到图形的变换矩阵来更改坐标系统的原点
            drawing.TranslateTransform(-(CInt(width) - textSize.Width) / 2, -(CInt(height) - textSize.Height) / 2)

            '绘制背景
            drawing.Clear(backColor)

            '为文本创建画笔
            Dim textBrush As Brush = New SolidBrush(textColor)

            '将文本绘制到图形的中心位置
            drawing.DrawString(text, font, textBrush, (CInt(width) - textSize.Width) / 2, (CInt(height) - textSize.Height) / 2)
            drawing.Save()
            Return img
        End Function
    End Class
End Namespace