"i worked it out.
Before i copy the file system to the NFS share in PXE server. i have to install nfsboot and nfsbooed first.
#apt-get install nfsboot nfsbooted
if we have not install these package, the copied kernal and initr file will not support the NFS mount.
that is the root cause the system can not find the root patition when boot from PXE.
we can say this is fixed."