小编tur*_*452的帖子

function abc() {
let child = document.querySelector("#child");
let parent = document.querySelector("#div_b");

parent.appendChild(child);
}

<div id="div_a" style="height:30px; width:30px; background-color:yellow;">
    <div id="child" class="new-box">
        <div style="width: 20px; height: 20px; background-color: green;"></div>
    </div>
  </div>

  <div id="div_b" style="height:30px; width:30px; background-color:red;">
  </div>
  
<button onclick="abc()">move</button>