I thought there is something wrong with my config, it turned out that the Casper NetSUS cannot config well with CentOS.
ExecStart=/usr/sbin/in.tftpd -s /srv/NetBoot/NetBootSP0
setsebool -P tftp_anon_write 1
setsebool -P tftp_home_dir 1
when you check, they are all on now. then I did a reboot :
getsebool -a | grep tftp
Links to read: