需要修改两个文件:

nano /etc/lightdm/lightdm.conf

autologin-user=root
autologin-user-timeout=0

nano /etc/pam.d/lightdm-autologin
 
#注释下面这行需要
auth  required pam_succeed_if.so user != root quiet_success
#auth  required pam_succeed_if.so user != root quiet_success