1、创建一个form表单

<form name=card action="" method=post>
    <input onkeydown="T1_onkeyup()" maxLength=1 name=T1>
    <input onkeydown="T2_onkeyup()" maxLength=1 name=T2>
    <input onkeydown="T3_onkeyup()" maxLength=1 name=T3>
    <input onkeydown="T4_onkeyup()" maxLength=1 name=T4> 
</form>

onkeydown：键盘按下事件
maxLength：input最大输入位数
可以添加type属性用来限制只输入数字，属性值为tel

2、引入jquery文件

<script src="https://code.jquery.com/jquery-3.0.0.min.js"></script>

3、编写js代码

<script>
    function T1_onkeyup() {
        if(document.card.T1.value.length==1){
            document.card.T2.focus();
        }
    } 
    function T2_onkeyup() {
        var oEvent = window.event;
        if (oEvent.keyCode == 8 && document.card.T2.value.length==0) {
            document.card.T1.focus();
        }
        if(document.card.T2.value.length==1){
            document.card.T3.focus();
        }
    }

    function T3_onkeyup() {
        var oEvent = window.event;
        if (oEvent.keyCode == 8 && document.card.T3.value.length==0) {
            document.card.T2.focus();
        }
        if(document.card.T3.value.length==1){
            document.card.T4.focus();
        }
    }
    function T4_onkeyup() {
        var oEvent = window.event;
        if (oEvent.keyCode == 8 && document.card.T4.value.length==0) {
            document.card.T3.focus();
        }
    }
</script>

oEvent.keyCode == 8 ：获取按下的键值为8（删除键）时回退上一个input框