我有点不太懂。
<SCRIPT LANGUAGE="JavaScript">
<!-- Begin
var arrItems1 = new Array();//声明一个数组对象
//P.S:javascript里数组可以不指定维数动态添加数据
var arrItemsGrp1 = new Array();//同上

//给数组赋值
arrItems1[3] = "Truck";
arrItemsGrp1[3] = 1;
arrItems1[4] = "Train";
arrItemsGrp1[4] = 1;
arrItems1[5] = "Car";
arrItemsGrp1[5] = 1;

arrItems1[6] = "Boat";
arrItemsGrp1[6] = 2;
arrItems1[7] = "Submarine";
arrItemsGrp1[7] = 2;

arrItems1[0] = "Planes";
arrItemsGrp1[0] = 3;
arrItems1[1] = "Ultralight";
arrItemsGrp1[1] = 3;
arrItems1[2] = "Glider";
arrItemsGrp1[2] = 3;

var arrItems2 = new Array();
var arrItemsGrp2 = new Array();

arrItems2[21] = "747";
arrItemsGrp2[21] = 0
arrItems2[22] = "Cessna";
arrItemsGrp2[22] = 0

arrItems2[31] = "Kolb Flyer";
arrItemsGrp2[31] = 1
arrItems2[34] = "Kitfox";
arrItemsGrp2[34] = 1

arrItems2[35] = "Schwietzer Glider";
arrItemsGrp2[35] = 2

arrItems2[99] = "Chevy Malibu";
arrItemsGrp2[99] = 5
arrItems2[100] = "Lincoln LS";
arrItemsGrp2[100] = 5
arrItems2[57] = "BMW Z3";
arrItemsGrp2[57] = 5

arrItems2[101] = "F-150";
arrItemsGrp2[101] = 3
arrItems2[102] = "Tahoe";
arrItemsGrp2[102] = 3

arrItems2[103] = "Freight Train";
arrItemsGrp2[103] = 4
arrItems2[104] = "Passenger Train";
arrItemsGrp2[104] = 4

arrItems2[105] = "Oil Tanker";
arrItemsGrp2[105] = 6
arrItems2[106] = "Fishing Boat";
arrItemsGrp2[106] = 6

arrItems2[200] = "Los Angelas Class";
arrItemsGrp2[200] = 7
arrItems2[201] = "Kilo Class";
arrItemsGrp2[201] = 7
arrItems2[203] = "Seawolf Class";
arrItemsGrp2[203] = 7

function selectChange(control, controlToPopulate, ItemArray, GroupArray)
{
//声明变量
var myEle ;
var x ;
// Empty the second drop down box of any choices
//清空controlToPopulate下拉列表的所有选项
for(var q=controlToPopulate.options.length;q>=0;q--)
controlToPopulate.options[q]=null;
//如果control下拉列表的名字为firstChoice
if (control.name == "firstChoice")
{
// Empty the third drop down box of any choices
//清空第3个下拉列表的选项
for (var q=myChoices.thirdChoice.options.length;q>=0;q--)
myChoices.thirdChoice.options[q] = null;
}
// ADD Default Choice - in case there are no values
//创建一个下拉列表的选项
myEle = document.createElement("option") ;
myEle.value = 0 ;//选项的值
myEle.text = "[SELECT]" ;//文本
//添加选项进入下拉列表
controlToPopulate.add(myEle) ;
//循环ItemArray选项数组
for ( x = 0 ; x < ItemArray.length ; x++ )
{
//如果GroupArray[x]的值等于下拉列表的value
if ( GroupArray[x] == control.value )
{
//同上面的代码
myEle = document.createElement("option") ;
myEle.value = x ;
myEle.text = ItemArray[x] ;
controlToPopulate.add(myEle) ;
}
}
}
// End -->
</script>
<form name=myChoices>
<table align="center">
<tr>
<td>
<SELECT id=firstChoice name=firstChoice onchange="selectChange(this, myChoices.secondChoice, arrItems1, arrItemsGrp1);">
<option value=0 SELECTED>[SELECT]</option>
<option value=1>Land</option>
<option value=2>Sea</option>
<option value=3>Air</option>
</SELECT>
</TD><TD>
<SELECT id=secondChoice name=secondChoice onchange="selectChange(this, myChoices.thirdChoice, arrItems2, arrItemsGrp2);">
</SELECT>
<SELECT id=thirdChoice name=thirdChoice>
</SELECT>
</TD>
</TR>
</TABLE>
</form>