我也在寻找这个，发现了这个

<!-- Place this tag in your head or just before your close body tag -->
<script type="text/javascript" src="https://apis.google.com/js/plusone.js">
  {parsetags: 'explicit'}
</script>

<!-- Place this tag where you want the +1 button to render -->
<g:plusone></g:plusone>

<!-- Place this render call where appropriate -->
<script type="text/javascript">gapi.plusone.go();</script>

gapi.plusone.go(); 放置此代码，按钮将出现