Edit in JSFiddle

<h4>默认flex-start</h4>
<ul>
  <li>1</li>
  <li>2</li>
  <li>3</li>
  <li>4</li>
</ul>

<h4>flex-end</h4>
<ul id="second">
  <li>1</li>
  <li>2</li>
  <li>3</li>
  <li>4</li>
</ul>

<h4>center</h4>
<ul id="third">
  <li>1</li>
  <li>2</li>
  <li>3</li>
  <li>4</li>
</ul>

<h4>space-between</h4>
<ul id="forth">
  <li>1</li>
  <li>2</li>
  <li>3</li>
  <li>4</li>
</ul>

<h4>space-around</h4>
<ul id="fifth">
  <li>1</li>
  <li>2</li>
  <li>3</li>
  <li>4</li>
</ul>

ul {
  display: flex;
  width: 400px;
  padding: 4px;
  border: 1px solid #ddd;
}
li {
  width: 50px;
  height: 50px;
  list-style:none;
  border: 1px solid #ddd;
  background: #eee;
}
li + li {
  margin-left: 10px;
}

ul#second{
  justify-content: flex-end;
}

ul#third {
  justify-content: center;
}

ul#forth {
  justify-content: space-between;
}

ul#fifth {
  justify-content: space-around;
}