* {
  margin: 0;
  padding: 0;
  font-family: "微软雅黑";
}

span {
  margin: 50px auto;
  display: block;
  width: 320px;
  height: 40px;
  background: #5a2dff;
  color: #fff;
  text-align: center;
  line-height: 40px;
  border-radius: 20px;
  cursor: pointer;
  position: relative;
  z-index: 0;
}

span:after {
  content: '';
  position: absolute;
  margin: auto;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  background: #fe6501;
  border-radius: 20px;
  z-index: -1;
  width: 100%;
  height: 40px;
  display: none;
}

span:hover:after {
  animation: ss 0.4s cubic-bezier(1,0,.22,.96);
  display: block;
}

@keyframes ss {
  0% {

    width: 0;
    height: 0
  }
  40% {
    width: 40px;
    height: 40px
  }
  100% {
    width: 100%;
    height: 40px;
  }
}