您需要先关闭其他折叠面板，然后再打开单击的折叠面板。

var acc = document.getElementsByClassName("accordion");
var i, j;

for (i = 0; i < acc.length; i++) {
  acc[i].addEventListener("click", function() {
    for (j = 0; j < acc.length; j++) {
      acc[j].classList.remove("active");
      acc[j].nextElementSibling.style.maxHeight = null;
    }
    if (!this.classList.contains("active")){
      this.classList.toggle("active");
      var panel = this.nextElementSibling;
      if (panel.style.maxHeight){
        panel.style.maxHeight = null;
      } else {
        panel.style.maxHeight = panel.scrollHeight + "px";
      } 
    }

  });
}

.accordion {
    background-color: #EA3F8A;
    color: #FFF;
    cursor: pointer;
    padding: 18px;
    width: 100%;
    border: none;
    text-align: left;
    outline: none;
    font-size: 15px;
    transition: 0.4s;
}

.active, .accordion:hover {
    background-color: #C11561;
}

.accordion:after {
    content: '\002B';
    color: #FFF;
    font-weight: bold;
    float: right;
    margin-left: 5px;
}

.active:after {
    content: "\2212";
}

.panel {
    padding: 0 18px;
    background-color: white;
    max-height: 0;
    overflow: hidden;
    transition: max-height 0.2s ease-out;
}

#accord {
    margin-bottom: 5.5em;
}

<button class="accordion">Section 1</button>
    <div class="panel">
    <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.</p>
    </div>
    
    <button class="accordion">Section 2</button>
    <div class="panel">
    <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.</p>
    </div>
    <button class="accordion">Section 2</button>
    <div class="panel">
    <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.</p>
    </div>

我一直觉得自己做 accordion 更容易：

$('.item-title').click(function() {
        $(this).parents().siblings().find('.item-body').slideUp();
        $(this).parents().siblings('.item').find('.item-open-close').removeClass('fa-minus').addClass('fa-plus');
        $(this).find('i').toggleClass('fa-minus fa-plus');
        $(this).siblings('.item-body').slideToggle();
    });

.item-title i {
float:right;
}

.item-title {
    background-color: #EA3F8A;
    color: #FFF;
    cursor: pointer;
    padding: 18px;
    border: none;
    text-align: left;
    outline: none;
    font-size: 15px;
    transition: 0.4s;
}
.item-body {
padding:18px;
}

<link href="https://stackpath.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css" rel="stylesheet"/>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.8.2/jquery.min.js"></script>
<div class="accordion">            
  <div class="item">
    <div class="item-title">
      Section 1<i class="fa item-open-close fa-plus"></i>
    </div>
    <div class="item-body" style="display: none;">
      This is content. This is content. This is content. This is content. This is content. This is content. This is content.   
    </div>  
  </div>
  <div class="item">
    <div class="item-title">
      Section 2<i class="fa item-open-close fa-plus"></i>
    </div>
    <div class="item-body" style="display: none;">
      This is content. This is content. This is content. This is content. This is content. This is content. This is content.   
    </div>  
  </div>
  <div class="item">
    <div class="item-title">
      Section 3<i class="fa item-open-close fa-plus"></i>
    </div>
    <div class="item-body" style="display: none;">
      This is content. This is content. This is content. This is content. This is content. This is content. This is content.   
    </div>  
  </div>
</div>
</div>

希望这有帮助：）

改进了@MayK的答案，以便可以关闭当前项目。

var acc = document.getElementsByClassName("accordion");
    var i, j;
    
    for (i = 0; i < acc.length; i++) {
      acc[i].addEventListener("click", function() {
        for (j = 0; j < acc.length; j++) {
        if(acc[j] != this) {
           acc[j].classList.remove("active");
           acc[j].nextElementSibling.style.maxHeight = null;
         }
        }
        if (!this.classList.contains("active")){
          this.classList.toggle("active");
          var panel = this.nextElementSibling;
          if (panel.style.maxHeight){
            panel.style.maxHeight = null;
          } else {
            panel.style.maxHeight = panel.scrollHeight + "px";
          } 
        }
    
      });
    }