.navbar{

width：100%；

margin：0 atuo；

}

<nav class="navbar navbar-default">

  <div class="container-fluid">//container-fluid这个类是自适应，container这个类是默认宽度，居中，老师没讲全，导航条要在最外层包裹<nav>

    <!-- Brand and toggle get grouped for better mobile display -->

    <div class="navbar-header">

      <button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#bs-example-navbar-collapse-1" aria-expanded="false">

        <span class="sr-only">Toggle navigation</span>

        <span class="icon-bar"></span>

        <span class="icon-bar"></span>

        <span class="icon-bar"></span>

      </button>

      <a class="navbar-brand" href="#">Brand</a>

    </div>

    <!-- Collect the nav links, forms, and other content for toggling -->

    <div class="collapse navbar-collapse" id="bs-example-navbar-collapse-1">

      <ul class="nav navbar-nav">

        <li class="active"><a href="#">Link <span class="sr-only">(current)</span></a></li>

        <li><a href="#">Link</a></li>

        <li class="dropdown">

          <a href="#" class="dropdown-toggle" data-toggle="dropdown" role="button" aria-haspopup="true" aria-expanded="false">Dropdown <span class="caret"></span></a>

          <ul class="dropdown-menu">

            <li><a href="#">Action</a></li>

            <li><a href="#">Another action</a></li>

            <li><a href="#">Something else here</a></li>

            <li role="separator" class="divider"></li>

            <li><a href="#">Separated link</a></li>

            <li role="separator" class="divider"></li>

            <li><a href="#">One more separated link</a></li>

          </ul>

        </li>

      </ul>

      <form class="navbar-form navbar-left" role="search">

        <div class="form-group">

          <input type="text" class="form-control" placeholder="Search">

        </div>

        <button type="submit" class="btn btn-default">Submit</button>

      </form>

      <ul class="nav navbar-nav navbar-right">

        <li><a href="#">Link</a></li>

        <li class="dropdown">

          <a href="#" class="dropdown-toggle" data-toggle="dropdown" role="button" aria-haspopup="true" aria-expanded="false">Dropdown <span class="caret"></span></a>

          <ul class="dropdown-menu">

            <li><a href="#">Action</a></li>

            <li><a href="#">Another action</a></li>

            <li><a href="#">Something else here</a></li>

            <li role="separator" class="divider"></li>

            <li><a href="#">Separated link</a></li>

          </ul>

        </li>

      </ul>

    </div><!-- /.navbar-collapse -->

  </div><!-- /.container-fluid -->

</nav>

用position:fixed 试试