Generally, there are two ways:

1, You can do this a variety of ways, but perhaps the most
straightforward is "ar -x packagename.deb". This will give two tarballs
control.tar.gz and data.tar.gz. The first is the information dpkg needs
to do a proper installation and configuration of the package, the
second is contains the binaries and data files.

2, You can use alien to transform *.deb to rpm package, and then using cpio to unpack it.
# rpm2cpio *.rpm | cpio -i -d -m