提供HTML说明了问题：您的content DIV不是title的子对象，因此$(".content", this)返回并清空jQuery对象。如果您不想重新排列HTML，我建议使用siblings()函数：https://api.jquery.com/siblings/#siblings-selector

$(document).ready(function() {
  $(".title", this).click(function() {
    console.log(this.tagName+" "+this.innerHTML);
    $(this).siblings(".content").slideToggle();
  });
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div class="container">
  <div class="title">im a title</div>
  <div class="content">this is a text</div>
</div>
<div class="container">
  <div class="title">im another title</div>
  <div class="content">and another text</div>
</div>