你不仅可以保持运行中的索引吗？

var _selectIndex = 0;

...code...
var newSelectBox = document.createElement("select");
newSelectBox.setAttribute("id","select-"+_selectIndex++);

编辑

经过进一步考虑，你实际上可能更喜欢对选择使用数组样式的名称。

例如

<select name="city[]"><option ..../></select>
<select name="city[]"><option ..../></select>
<select name="city[]"><option ..../></select>

然后，例如在php的服务器端：

$cities = $_POST['city']; //array of option values from selects

编辑2回应OP评论

可以使用DOM方法动态创建选项，如下所示：

var newSelectBox = document.createElement("select");
newSelectBox.setAttribute("id","select-"+_selectIndex++);

var city = null,city_opt=null;
for (var i=0, len=cities.length; i< len; i++) {
    city = cities[i];
    var city_opt = document.createElement("option");
    city_opt.setAttribute("value",city);
    city_opt.appendChild(document.createTextNode(city));
    newSelectBox.appendChild(city_opt);
}
document.getElementById("example_element").appendChild(newSelectBox);

假设cities数组已经存在

或者，你可以使用innerHTML方法.....

var newSelectBox = document.createElement("select");
newSelectBox.setAttribute("id","select-"+_selectIndex++);
document.getElementById("example_element").appendChild(newSelectBox);

var city = null,htmlStr="";
for (var i=0, len=cities.length; i< len; i++) {
    city = cities[i];
    htmlStr += "<option value='" + city + "'>" + city + "</option>";
}
newSelectBox.innerHTML = htmlStr;