您的i变量缺少var声明。所有循环都使用相同的计数器，这意味着嵌套循环会严重中断。我的建议：

var a = [];
function FormElement(type, value) {
    this.type = type;
    this.value = value;
}

FormElement.prototype.toString = function toStr(){
    if (this.type == "select") {
        var arr = this.value.split("/");
        for (var i = 0; i < arr.length; i++) {
            arr[i] = "<option>" + arr[i] + "</option>"
        }
        return "<select>" + arr.join("") + "</select><br />";
    } else
        return "<input type = '" + this.type + "' value = '" + this.value + "' /><br />";
};

function addObj(type) {
    var form = parent.frames["left"].document.form1; 
    var sel = form.q.value;
    for (var i = 0; i < sel; i++)
        a.push(new FormElement(type, form.caption_text.value);
    paint();
    form.caption_text.value = "";
}

function paint() {
    var doc = parent.frames["right"].document;
    doc.open();
    doc.write(a.join("\n"));
    doc.close();
}