可以参考如下示例代码，实现动态向表单中添加和删除 select 下拉框：
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>动态添加和删除下拉框</title>
<link rel="stylesheet" href="https://cdn.staticfile.org/layui/2.5.6/css/layui.css">
</head>
<body>
<div class="layui-container">
<form class="layui-form" action="">
<div id="select-container">
<div class="layui-form-item">
<label class="layui-form-label">下拉框1</label>
<div class="layui-input-inline">
<select name="select1" lay-filter="select1">
<option value=""></option>
</select>
</div>
<div class="layui-input-inline">
<button type="button" class="layui-btn layui-btn-danger delete-select"><i class="layui-icon layui-icon-delete"></i> 删除</button>
</div>
</div>
</div>
<div class="layui-form-item">
<div class="layui-input-block">
<button type="button" class="layui-btn" id="add-select"><i class="layui-icon layui-icon-add-1"></i> 添加下拉框</button>
</div>
</div>
</form>
</div>
<script src="https://cdn.staticfile.org/jquery/3.5.1/jquery.min.js"></script>
<script src="https://cdn.staticfile.org/layui/2.5.6/layui.js"></script>
<script>
// layui初始化
layui.use(['form'], function(){
var form = layui.form;
// 添加下拉框按钮的点击事件
$('#add-select').on('click', function(){
// 获取已有下拉框个数
var count = $('#select-container .layui-form-item').length;
// 添加新的下拉框
var selectHtml = '<div class="layui-form-item">'
+ '<label class="layui-form-label">下拉框' + (count + 1) + '</label>'
+ '<div class="layui-input-inline">'
+ '<select name="select' + (count + 1) + '" lay-filter="select' + (count + 1) + '">'
+ '<option value=""></option>'
+ '</select>'
+ '</div>'
+ '<div class="layui-input-inline">'
+ '<button type="button" class="layui-btn layui-btn-danger delete-select"><i class="layui-icon layui-icon-delete"></i> 删除</button>'
+ '</div>'
+ '</div>';
$('#select-container').append(selectHtml);
// 重新渲染下拉框
form.render('select');
});
// 删除下拉框按钮的点击事件
$('#select-container').on('click', '.delete-select', function(){
$(this).closest('.layui-form-item').remove();
});
});
</script>
</body>
</html>
运行该示例代码，即可看到一个表单中有一个下拉框和一个添加下拉框的按钮，点击按钮可以动态添加下拉框，并且每个下拉框旁边都有一个删除按钮，点击删除按钮即可删除对应的下拉框。
