function showIframeH(){
  var parentWin = parent.document.getElementById("test");
  if(!parentWin) return false;
  var sub = parentWin.contentWindow.document.getElementById("test2");
  if(!sub) return false;
  var thirdHeight = sub.contentWindow.document.body.offsetHeight; //第三层 body 对象
  sub.height = thirdHeight; //设置第二层 iframe 的高度
  var secondHeight = x.contentWindow.document.body.offsetHeight; //第二层 body 对象
  x.height = secondHeight; //设置第一层 iframe 的高度
  //alert(secondHeight);
  //alert('body: ' + x.contentDocument.body.offsetHeight + ' div:' + thirdHeight);
}

<script>
	function mainFrameLoaded(t) 
	{
		//debugger;
		//alert(t.contentWindow.document.body.offsetHeight);
  t.style.height = t.contentWindow.document.body.offsetHeight + 'px';
	}
</script>