function changeselect(selectValue)
{
document.form.Shop_Road.length =-1;
document.form.Shop_Road.options.add(new Option('请选择小类',''));
for (i=0; i<subcat.length; i++)
{
if (subcat[i][0] == selectValue)
{
document.form.Shop_Road.options.add(new Option(subcat[i][1], subcat[i][2]));
}
}
试试看。