<table width="270" border="0" cellspacing="0">
        <form name="form1" action="products.asp" method="post">
            <tr>
                <td width="186">                 
     <select name="bigclassname" onChange="redirec(document.form1.bigclassname.options.selectedIndex)">
        <option>请选择</option>
        <option value="1">投影机</option>
        <option value="2">视频展台</option>
        <option value="3">电子白板</option>
        <option value="4">会议音响</option>
      </select>
      <select name="smallclassname">
            <option value="请选择" selected>请选择</option>
        </select>
        <script language="javascript">
            //获取一级菜单长度
            var select1_len = document.form1.bigclassname.options.length;
            var select2 = new Array(select1_len);
            //把一级菜单都设为数组
            for (i=0; i<select1_len; i++)
            { select2[i] = new Array();}
            //定义基本选项
            select2[0][0] = new Option("请选择", " ");
            
            select2[1][0] = new Option("东芝系列", "94");
            select2[1][1] = new Option("夏普系列", "96");
            select2[1][2] = new Option("NEC系列", "97");
            select2[1][3] = new Option("三洋系列", "119");
            select2[1][4] = new Option("索尼系列", "120");
            select2[1][5] = new Option("爱普生系列", "121");
            select2[1][6] = new Option("松下系列", "122");
            select2[1][7] = new Option("日立系列", "123");
            select2[1][8] = new Option("其他系列", "133");
            　
            
            select2[2][0] = new Option("圆刚系列", "98");
            select2[2][1] = new Option("高科系列", "99");  
            select2[2][2] = new Option("久远系列", "100");
            select2[2][3] = new Option("其他系列", "134");
                
            select2[3][0] = new Option("东芝系列", "101");
            select2[3][1] = new Option("巨龙系列", "102");
            select2[3][2] = new Option("其他系列", "135");  
            
            select2[4][0] = new Option("松下系列", "103");
            select2[4][1] = new Option("其他系列", "124");


            //联动函数
            function redirec(x)
            {
            var temp = document.form1.smallclassname;
            for (i=0;i<select2[x].length;i++)
            { temp.options[i]=new Option(select2[x][i].text,select2[x][i].value);}
            temp.options[0].selected=true;
            }
        </script>                </td>
                <td width="80">
                <input type="image" name="imageField" id="imageField" src="images/sousuo.jpg"  /></td>
            </tr>
      
        </table>

这个是一个可以，用的就是再多放几个的时候就好用了

这个是一个可以，就是再多放几个的时候就不好用了