sudo nano /usr/share/lightdm/lightdm.conf.d/20-lubuntu.conf

[Seat:*]
user-session=Lubuntu ##原有
autologin-user=username
autologin-user-timeout=0

lcpc:/usr/share/lightdm/lightdm.conf.d$ ls
20-lubuntu.conf             50-guest-wrapper.conf
50-disable-guest.conf       50-xserver-command.conf
50-disable-log-backup.conf  60-lightdm-gtk-greeter.conf
50-greeter-wrapper.conf