public bool ExportDataGridview(DataGridView gridView, bool isShowExcele)
        {
            if (gridView.Rows.Count == 0)
            {
                return false;
            }
            //建立Excel对象
            Excel.Application excel = new Excel.Application();
            excel.Application.Workbooks.Add(true);
            excel.Visible = isShowExcele;
            int rowCount = gridView.Rows.Count;
            int colCount = gridView.Columns.Count;
            object[,] dataArray = new object[rowCount, colCount];
            for (int i = 0; i < gridView.ColumnCount; i++)
            {
                dataArray[0, i] = gridView.Columns[i].HeaderText;
            }
            for (int i = 0; i < rowCount-1; i++)
            {
                for (int j = 0; j < colCount; j++)
                {
                    if (gridView[j, i].Value.GetType() == typeof(string))
                    {
                        dataArray[i+1, j] = "'" + gridView[j,i].Value.ToString();//在字符型的变量前加'
                    }
                    else
                    {
                        dataArray[i+1, j]  = gridView[j, i].Value.ToString();
                    }
                }
            }
            excel.get_Range("A1", excel.Cells[rowCount, colCount]).Value2 = dataArray;
            excel.Cells.EntireColumn.AutoFit();
            return true;
        }

        private void button1_Click(object sender, EventArgs e)
        {
            this.ExportDataGridview(dataGridView1, true);
        }

------解决方案--------------------
写入单元格的时候，前面加一个单引号

------解决方案--------------------
 导出用 XML 然后改成XLS就行了.
   //---将表格写成XML文件---不用样式表
        public static void ExportXls(DataGridView dgv,string filename)
       {
           try
           {
               filename = filename.Replace("xls", "xml"); ;
               System.Xml.XmlTextWriter writer = null;
               writer = new System.Xml.XmlTextWriter(filename, null);
               ////为使文件易读，使用缩进
               writer.Formatting = System.Xml.Formatting.Indented;

               ////写XML声明
               writer.WriteStartDocument(true);

               //写入注释
              // writer.WriteComment("xml To xls ＝ xml文件转换成xls文件");

               writer.WriteStartElement("Workbook");
               writer.WriteAttributeString("xmlns:msxsl", "urn:schemas-microsoft-com:xslt");
               writer.WriteAttributeString("xmlns:user", "urn:my-scripts");
               writer.WriteAttributeString("xmlns", "urn:schemas-microsoft-com:office:spreadsheet");
               writer.WriteAttributeString("xmlns:o", "urn:schemas-microsoft-com:office:office");
               writer.WriteAttributeString("xmlns:x", "urn:schemas-microsoft-com:office:excel");
               writer.WriteAttributeString("xmlns:ss", "urn:schemas-microsoft-com:office:spreadsheet");
               writer.WriteAttributeString("xmlns:html", "http://www.w3.org/TR/REC-html40");
               //--单元格格式---
               writer.WriteStartElement("Styles");
               writer.WriteStartElement("Style");
               writer.WriteAttributeString("ss:ID", "s0");
               //--以下是对齐样式
               writer.WriteStartElement("Alignment");