wget https://mirrors.aliyun.com/centos/6.9/os/x86_64/images/pxeboot/initrd.img
wget https://mirrors.aliyun.com/centos/6.9/os/x86_64/images/pxeboot/vmlinuz

mv vmlinuz /boot/vmlinuz.cent.pxe
mv initrd.img /boot/initrd.img.cent.pxe

vi /boot/grub/grub.conf

title Centos Install (PXE)
    root (hd0,0)
    kernel /boot/vmlinuz.cent.pxe
    initrd /boot/initrd.img.cent.pxe