您可以这样做，只需将值添加为国家/地区的代码，如对于US，选项的值应为1
您HTML应该是，

<select id="country">
   <option value="">Select Country</option>
   <option value="1">US</select>
   <option value="91">IN</select>
</select><br/>
Code: <input type="text" id="code" />

Jquery

$(function(){
   $('#country').on('change',function(){
       $('#code').val(this.value);// changing the code textbox value by current country value
   });
});

于飞：

<select id="country">
    <option value="">Select Country</option>
    <option value="1">US</option>
    <option value="+44 ">UK</option>
</select>
<input id="phone" type="text" name="phone">

查询：

$(function() {
    $('#country').on('change', function() {
        $('#phone').val($(this).val());
    });
});

<select id="country">
   <option value="">Select Country</option>
   <option value="1">US</option>
   <option value="91">IN</option>
   </select>
<br><br>
Code: <input type="text" id="code" />

$(function() {
    $('#country').on('change', function() {
        $('#phone').val($(this).val());
    });
});`enter code here`