添加此样式：
.inner { flex-direction: column;}这告诉flexbox在行而不是列中显示其子级。（我知道，很奇怪，对吧？）
更新的代码段：
.container { height: 200px; width: 200px; background: cornflowerblue; position: relative;}.inner { height: 100%; position: absolute; left: 0; width: 100%; top: 0; display: -webkit-box; display: -moz-box; display: -ms-flexbox; display: -webkit-flex; display: flex; align-items: center; justify-content: center; flex-direction: column;}.square { width: 30px; height: 30px; background: tomato; display: block;}<div > <div > <div ></div> <p>some text</p> </div></div>
