<select id="selclass" name="selclass">
  <option selected="" value="sno">按证书编号查询</option>
  <option value="com">按单位名称查询</option>
  <option value="pro">按产品名称查询</option>
  <option value="hangye">按六大行业查询</option>
</select>
<input type="text" maxlength="50" size="25" id="seltext" class="INPUT" name="seltext" />
<input width="77" type="button" height="24" border="0" id="btnSearch" value="查询" onclick="searchByKey()" /><br />
<iframe src="" id="search" width="100%" frameborder="0"</iframe>

<script type="text/javascript">
function searchByKey(){
    var selclass = document.getElementById("selclass").value;
    var seltext = document.getElementById("seltext").value;
    var url = 'http://www.china-fr.com/zs.asp?selclass=' + selclass + '&seltext=' + seltext;
    document.getElementById("search").src=url;
}
</script>