U盘坏了，用个下面的方法安装

安装配置dhcp

yum -y install dhcp tftp-server

修改如下，网段改为你自己的网段

vim /etc/dhcp/dhcpd.confsubnet 172.16.1.0 netmask 255.255.255.0 {range 172.16.1.100 172.16.1.200;option subnet-mask 255.255.255.0;default-lease-time 21600;max-lease-time 43200;next-server 172.16.1.61; #这里ip为tftp服务器的ipfilename "/pxelinux.0";
}

安装httpd

yum -y install httpd
sed -i "277i ServerName 127.0.0.1:80" /etc/httpd/conf/httpd.conf

挂在CentOS7 镜像

mkdir /var/www/html/CentOS-7
mount /dev/cdrom /var/www/html/CentOS-7/

准备pxelinux.0文件

yum -y install syslinux
cp /usr/share/syslinux/pxelinux.0 /var/lib/tftpboot/

复制isolinux所有文件至tftpboot

cp -a /var/www/html/CentOS-7/isolinux/* /var/lib/tftpboot/

复制pxe配置文件

mkdir -p /var/lib/tftpboot/pxelinux.cfg
cp /var/www/html/CentOS-7/isolinux/isolinux.cfg /var/lib/tftpboot/pxelinux.cfg/default

编辑pxe配置文件

vim /var/lib/tftpboot/pxelinux.cfg/defaultdefault  vesamenu.c32
prompt 0
timeout 300
ONTIMEOUT localmenu title ----------------------- Install The Centos 7 Linux -----------------------------label 1
menu label ^1) Install CentOS 7 x64 kernel vmlinuzappend initrd=initrd.img method=http://172.16.1.1/CentOS-7 devfs=nomountlabel 2
menu label ^2) Install CentOS 7 x64 with VNCkernel vmlinuzappend  initrd=initrd.img method=http://172.16.1.1/CentOS-7 devfs=nomount inst.vnclabel 3
menu label ^3) Auto Install CentOS 7 x64kernel vmlinuzappend initrd=initrd.img ks=http://172.16.1.1/ks/CentOS-7-ks.cfg ksdevice=em1label 4
menu label ^4) Boot from local drivelocalboot 0xffff

启动服务

/etc/init.d/dhcpd start
/etc/init.d/httpd start
/etc/init.d/xinetd start
chkconfig tftp on

如果选择2使用VNC方式，进入之后，访问地址在显示器上有输出,使用VNC客户端连接 IP:1 登陆安装系统