将flex-grow:1；添加到.content中

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.container{
  display: flex;
  flex-direction: column;
  width: 100%;
  height: 100vh;
  background-color: gray;
}
.header{
  width: 100%;
  height: 80px;
  background-color: red;
}

.content{
  align-self: stretch;
  width: 100%;
  background-color: blue;
  flex-grow: 1; /* Add this line */
}

<div class="container">
  <div class="header">
    Header
  </div>
  <div class="content">
    Be filled the rest part
  </div> 
</div>