解决方案一 (利用父级的 :after 占位)

<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
	</head>
	<style type="text/css">
		li {
			list-style: none;
		}

		.List {
			width: 100%;
			box-sizing: border-box;
			display: flex;
			justify-content: space-between;
			flex-wrap: wrap;
			padding: 10px;
		}
		.List:after{
			content: "";
			flex: auto;
		}

		.item {
			/* 宽度是固定好的 */
			width: 220px;
			height: 60px;
			background-color: azure;
			margin-bottom: 10px;
			border: 1px solid red;
			/*这个数值需要自己去调*/
			margin-right: 17px;
		}
		.item:nth-child(5n){
			/* 尽量让item在list中居中，两边都没有margin */
			margin-right: 0;
		}
		
	</style>
	<body>
		<div id="" style="width: 1200px;margin: auto;background-color: #cccccc;height: 500px;">
			<ul class="List">
				<li class="item"></li>
				<li class="item"></li>
				<li class="item"></li>
				<li class="item"></li>
				<li class="item"></li>
				<li class="item"></li>
				<li class="item"></li>
				<li class="item"></li>
				<li class="item"></li>
				<li class="item"></li>
				<li class="item"></li>
				<li class="item"></li>
				<li class="item"></li>
				
			</ul>
		</div>

	</body>
</html>

重要代码：要加after以及每个item的margin-right

.List:after{
		content: "";
		flex: auto;
	}

解决方案二(利用:last-child和:nth-child()占位)

<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
	</head>
	<style type="text/css">
    *{
      margin: 0;
      padding: 0;
    }
		.container {
    overflow: hidden;
}
.container .content {
    display: flex;
    flex-wrap: wrap;
    justify-content: space-between;
    margin-left: -2%;
}
.container .item {
    color: #fff;
    line-height: 1.5em;
    text-align: center;
    width: 23%;
    height: 100px;
    background-color: #2193b0;
    margin-top: 2%;;
    margin-left: 2%;
}
.container .item:last-child:nth-child(4n-2) {/* 如果是2个元素 */
    margin-right: calc(100% - 46% - 4%);
}
.container .item:last-child:nth-child(4n-1) {/* 如果是3个元素 */
    margin-right: calc(100% - 69% - 6%);
}
		
	</style>
	<body>
    <div class="container">
      <div class="content"> 
          <div class="item">item 1</div>
          <div class="item">item 2</div>
          <div class="item">item 3</div>
          <div class="item">item 4</div>
          <div class="item">item 5</div>
          <div class="item">item 6</div>
          <div class="item">item 7</div>
          <div class="item">item 8</div>
          <div class="item">item 9</div>
          <div class="item">item 10</div>
      </div>
  </div>
  

	</body>
</html>

解决方案三(补位添加节点法，这种方案适用于多种排列方式)


<div class="flex">
    <div class="flex-item" v-for="item in len">列表</div>
    <div class="list" v-for="item in (row - len % row)" v-if="len%row > 0"></div>
</div>

data(){
    return {
        len : 14, // 一共有多少个
        row: 4    // 有几行（实际情况，行数应该计算得来，这里偷懒了）
    }
}
.flex{
    display: flex;
    flex-wrap: wrap;
    justify-content:space-between;
    justify-items: center;
    text-align: justify;
}

.flex-item{
    width:20%;
    border:1px solid #ff6600;
    margin-bottom: 10px;
    padding: 10px 5px;
    display: flex;
    justify-content: center;
}

.list{
    content: '';
    width: 20%;
    border:1px solid transparent;
    padding: 5px;
    overflow: hidden;
}