<div class="container w320">
  <h5>320px mobile width</h5>
  <div class="void320">
    <img src="http://ockcz5ezf.bkt.clouddn.com/20180203151767016835794.png" alt="">
  </div>
</div>

<div class="container w375">
  <h5>375px mobile width</h5>
  <div class="void375">
    <img src="http://ockcz5ezf.bkt.clouddn.com/20180203151767016835794.png" alt="">
  </div>
</div>

html {
  font-size: 20px;
}
.container {
  display: inline-block;
}
.w320 {
  width: 320px;
}

.w375 {
  width: 375px;
}

.void320 {
  position: relative;
  width: 100%;
  padding-bottom: 50%;
}
.void375 {
  position: relative;
  width: 100%;
  padding-bottom: 50%;
}
img {
  position: absolute;
  left: 0;
  top: 0;
  width: 100%;
  height: 100%;
}