Hi,
[quote user="jianping ding"]how can i use CGAL to do this job?[/quote]
I used CGAL as an example only, because it's I guess one of the most famous open source free 2D and 3D algorithm library. The main message of what I wrote is "There is no the functionality required, search for external library / solution."
CGAL consists from hundreds of different algorithms, so it's up to you to search for the best one or to ask in CGAL support forum (stackoverflow is used for this purpose). I don't expect there will be anything like "mesh_unfold()" function available, because unfolding is a result of other functions and can be implemented in different ways depending on requirements and mesh parameters.
Use Internet, I was able quickly find some recommendations (e.g. this one) or you can get some inspiration in oter open source software that implements this functionality (I guess e.g. Blender offers unfolding).
So the summary is it's not simple task and probably requires to study proper algorithm before the implementation can be done.
With regards,
Jan