我不确定css / scss，但是如果您喜欢使用jquery，可以这样做

im not sure about css/scss but if you prefer with jquery you can do it like this

$(document).ready(function() {
  $(".second_div").css({
    'width': ($(".first_div").width() + 'px')
  });
});

.second_div,
.first_div {
  border: 1px solid black;
  padding: 2px 4px;
  display: inline-block;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
<div class="first_div">First Div some text</div>
<div class="second_div">Second.width </div>