To expand a node, it's easy with Mouse by clicking "+".
Howerver, I wonder how to do that with code.
1.
 FlyTreeView1.SelectedNode.expand();//???????????
///No,nothing happened
2.
  FlyTreeView1.ExpandLevel=5;//suppose that the node's level=5
  The result is that not only one node but also all nodes with same level are expanded!
  
How to expand only branch?