如何将ajax（xmlhttprequest）转换为字符串？

You could use .each():

$xml.find("Coupon").each(function() {
    var id = $(this).find("CouponID").text(),
        rewardItem = $(this).find("RewardItem").text();

    alert(id);
    alert(rewardItem);
});

Basically, it's iterating over each Coupon element and finding the CouponID and RewardItem elements inside.