aspx

<asp:GridView ID="GV" runat="server" OnRowCreated="GV_RowCreated">
</asp:GridView>

aspx.cs

protected void GV_RowCreated(object sender, GridViewRowEventArgs e)
        {
            // 檢查是不是標題列
            if (e.Row.RowType == DataControlRowType.Header)
            {
                // 建立自訂的標題
                GridView gv = (GridView)sender;
                TableCellCollection tcHeader = e.Row.Cells;
                tcHeader.Clear();//清除自動生成的表頭


                GridViewRow gvRow1 = Header_Row1();
                GridViewRow gvRow2 = Header_Row2();
 

                // 把這一列加到最上面
                gv.Controls[0].Controls.AddAt(0, gvRow2);
                gv.Controls[0].Controls.AddAt(0, gvRow1);

            }
        }

        private  GridViewRow Header_Row1()
        {
            GridViewRow gvRow = new
               GridViewRow(0, 0, DataControlRowType.Header,
               DataControlRowState.Insert);

            // 增加 Department 欄位 
            TableCell tc1 = new TableCell();
            tc1.Text = "Department";
            tc1.ColumnSpan = 2;   // 跨兩欄
            tc1.RowSpan = 2;   // 跨兩欄
            Header_置中_粗體(tc1);
            gvRow.Cells.Add(tc1);

            // 增加 Employee 欄位
            TableCell tc2 = new TableCell();
            tc2.Text = "Employee";
            tc2.ColumnSpan = 3;   // 跨三欄
            Header_置中_粗體(tc2);
            gvRow.Cells.Add(tc2);
            return gvRow;
        }
              
        private GridViewRow Header_Row2()
        {
            GridViewRow gvRow = new
               GridViewRow(0, 0, DataControlRowType.Header,
               DataControlRowState.Insert);

            
            TableCell tc1 = new TableCell();
            tc1.Text = "header1";
            tc1.ColumnSpan = 1;
            Header_置中_粗體(tc1);
            gvRow.Cells.Add(tc1);

           
            TableCell tc2 = new TableCell();
            tc2.Text = "header2";
            tc2.ColumnSpan = 1;
            Header_置中_粗體(tc2);
            gvRow.Cells.Add(tc2);

            TableCell tc3 = new TableCell();
            tc3.Text = "header3";
            tc3.ColumnSpan = 1;
            Header_置中_粗體(tc3);
            gvRow.Cells.Add(tc3);

            return gvRow;
        }

        private static void Header_置中_粗體(TableCell tc2)
        {
            tc2.HorizontalAlign = HorizontalAlign.Center;
            tc2.Font.Bold = true;
            tc2.Font.Size = 9;
        }

ref
https://dolphins11.pixnet.net/blog/post/404457859