编辑：万一其他人遇到这个问题，我相信OP的解决方案是他没有导入
popper.js。
检查jQuery和所有相关的Bootstrap组件是否存在。还要检查控制台，并确保没有错误。
<!DOCTYPE html><html lang="en"><head> <!-- Required meta tags --> <meta charset="utf-8"> <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no"> <!-- Bootstrap CSS --> <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta/css/bootstrap.min.css" integrity="sha384-/Y6pD6FV/Vv2HJnA6t+vslU6fwYXjCFtcEpHbNJ0lyAFsXTsjBbfaDjzALeQsN6M" crossorigin="anonymous"></head><body> <nav > <a href="#">Navbar</a> <button type="button" data-toggle="collapse" data-target="#navbarNavDropdown" aria-controls="navbarNavDropdown" aria-expanded="false" aria-label="Toggle navigation"> <span ></span> </button> <div id="navbarNavDropdown"> <ul > <li > <a href="#">Home <span >(current)</span></a> </li> <li > <a href="#">Features</a> </li> <li > <a href="#">Pricing</a> </li> <li > <a href="http://example.com" id="navbarDropdownMenulink" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false"> Dropdown link </a> <div aria-labelledby="navbarDropdownMenulink"> <a href="#">Action</a> <a href="#">Another action</a> <a href="#">Something else here</a> </div> </li> </ul> </div> </nav> <!-- Optional Javascript --> <!-- jQuery first, then Popper.js, then Bootstrap JS --> <script src="https://pre.jquery.com/jquery-3.2.1.slim.min.js" integrity="sha384-KJ3o2DKtIkvYIK3UENzmM7KCkRr/rE9/Qpg6aAZGJwFDMVNA/GpGFF93hXpG5KkN" crossorigin="anonymous"></script> <script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.11.0/umd/popper.min.js" integrity="sha384-b/U6ypiBEHpOf/4+1nzFpr53nxSS+GLCkfwBdFNTxtclqqenISfwAzpKaMNFNmj4" crossorigin="anonymous"></script> <script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta/js/bootstrap.min.js" integrity="sha384-h0AbiXch4ZDo7tp9hKZ4TsHbi047NrKGLO3SEJAg45jXxnGIfYzk4Si90RDIqNm1" crossorigin="anonymous"></script></body></html>
