你可以给予model-content一个相对位置，modal-footer的绝对位置，bottom为0 px;
试试用

.modal-content {
  height: 100%;
  border-radius: 0;
  position:relative;
}

.modal-footer {
  border-radius: 0;
  bottom:0px;
  position:absolute;
  width:100%;
}

小提琴：http://jsfiddle.net/1fh2n5y3/

一切正常，我的朋友。
Jsfiddle

.modal-dialog {
  width: 100%;
  height: 100%;
  padding: 0;
}

.modal-content {
  height: 100%;
  border-radius: 0;
}

.modal-footer {
  position: absolute;
  bottom: 0;
  width: 100%;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<link href="http://netdna.bootstrapcdn.com/bootstrap/3.1.1/css/bootstrap.min.css" rel="stylesheet"/>
<script src="http://netdna.bootstrapcdn.com/bootstrap/3.1.1/js/bootstrap.min.js"></script>
<!-- Button trigger modal -->
<button class="btn btn-primary btn-lg" data-toggle="modal" data-target="#myModal">
  Launch demo modal
</button>

<!-- Modal -->
<div class="modal fade" id="myModal" tabindex="-1" role="dialog" aria-labelledby="myModalLabel" aria-hidden="true">
  <div class="modal-dialog">
    <div class="modal-content">
      <div class="modal-header">
        <button type="button" class="close" data-dismiss="modal" aria-hidden="true">&times;</button>
        <h4 class="modal-title" id="myModalLabel">Modal title</h4>
      </div>
      <div class="modal-body">
        ...
      </div>
      <div class="modal-footer">
        <button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
        <button type="button" class="btn btn-primary">Save changes</button>
      </div>
    </div>
  </div>
</div>

将这几行添加到模式页脚Fiddle默认情况下，模式内容具有位置相对性。我使用！important来在类modal-dialog上否决css。如果你有separet css文件，那么你不需要使用！很重要

.modal-dialog {

  width:100% !important;
  height: 100%;
  margin: 0 auto !important;
  padding:20px 20px;
}

.modal-content {
  height: 100%;
  border-radius: 0;
}

.modal-footer{
position:absolute;
bottom:0;
width:100%;
}

<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-BVYiiSIFeK1dGmJRAkycuHAHRg32OmUcww7on3RYdg4Va+PmSTsz/K68vbdEjh4u" crossorigin="anonymous">
<script
  src="https://code.jquery.com/jquery-3.1.1.min.js"
  integrity="sha256-hVVnYaiADRTO2PzUGmuLJr8BLUSjGIZsDYGmIJLv2b8="
  crossorigin="anonymous"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js" integrity="sha384-Tc5IQib027qvyjSMfHjOMaLkfuWVxZxUPnCJA7l2mCWNIpG9mGCD8wGNIcPD7Txa" crossorigin="anonymous"></script>
<!-- Button trigger modal -->
<button class="btn btn-primary btn-lg" data-toggle="modal" data-target="#myModal">
  Launch demo modal
</button>

<!-- Modal -->
<div class="modal fade" id="myModal" tabindex="-1" role="dialog" aria-labelledby="myModalLabel" aria-hidden="true">
  <div class="modal-dialog">
    <div class="modal-content">
      <div class="modal-header">
        <button type="button" class="close" data-dismiss="modal" aria-hidden="true">&times;</button>
        <h4 class="modal-title" id="myModalLabel">Modal title</h4>
      </div>
      <div class="modal-body">
        ...
      </div>
      <div class="modal-footer">
        <button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
        <button type="button" class="btn btn-primary">Save changes</button>
      </div>
    </div>
  </div>
</div>

如果您使用angular和ngbootstrap库并使用

ngbModal.open(MyComponent);

你可以把它放在你的组件里

@HostBinding('class') class = 'd-flex flex-column h-100';

并将modal-body或flex下的其他元素设置为flex-grow-1