您需要为此更新if条件.如果该值等于或大于(li length) - 1，则应从头开始.

You need to update the if condition for that.If the value is equal or greater than (li length) - 1 then it should start from the beginning.

var i = 2,
  $ul = $('#content ul'),
  int = setInterval(function() {
      $('li', $ul).slideUp();
      $('li' + (i == -1 ? '' : ':gt(' + i + ')') + ':lt(3)', $ul).slideDown();
      i += 3;
      if (i + 1 >= $('li', $ul).length) i = -1;
      // ----^----
    },
    2000);

#content ul li:nth-child(n+4) {
  display: none
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
<div id="content">
  <ul>
    <li>1</li>
    <li>2</li>
    <li>3</li>
    <li>4</li>
    <li>5</li>
    <li>6</li>
    <li>7</li>
    <li>8</li>
    <li>9</li>
  </ul>
</div>