- Go to System Settings…Brightness and Lock:
- The default screensaver idle time is 10 minutes, and the screen is locked once the screensaver activates:
- You can adjust the idle time (or disable the screensaver), and also disable the lock:
- Simply close Brightness and Lock to apply the new settings.
resource : https://askubuntu.com/questions/177348/how-do-i-disable-the-screensaver-lock/177353#177353