/// <summary>
        /// 执行绑定dropdownlist方法
        /// </summary>
        /// By Lifuyun
        /// <param name="strsql">要执行的SQL语句</param>
        /// <param name="tablename">要执行的SQL语句中的表名</param>
        /// <param name="strtext">DropDownList显示出来的文本</param>
        /// <param name="strvalue">DropDownList隐藏的对象</param>
        /// <param name="ddl">DropDownList对象</param>
        public void ConnectDDL(string strsql, string tablename, string strtext, string strvalue, DropDownList ddl)
        {
            OpenConnection();
            SqlCommand cmd = new SqlCommand(strsql, _conn);
            SqlDataReader dr = cmd.ExecuteReader();
            //将数据添加到 DropDownList
            while (dr.Read())
            {
                ddl.Items.Add(new ListItem(dr[strtext].ToString(), dr[strvalue].ToString()));
            }
            dr.Close();
            cmd.Dispose();
            ColseConnection();
        }

------解决方案--------------------
你可以试试如下:
DropDownList drp1 = ((DropDownList)gvPerson.Rows[e.NewEditIndex].FindControl("drp1"));
drp1.SelectedIndex = drp1.Items.IndexOf(drp1.Items.FindByText(dt.Rows[0]["f1"].ToString()));