I tried this before but I can't do it now.Look like it's a special log in screen I get,it's useful,so I wanna do it again,I rarely remember how to what is it.like use fast user switching,I don't need password to log in.
Your question is not very clear but I think you are asking how to turn on "Automatic Login" which will go you in to the computer when it starts without asking for a password.
In System Preferences select "Users & Groups". Click on the padlock at the bottom left of the window and you will be asked for your password. Once you have done that click on "Login Options" at the bottom of the left hand pane.
Now in the right hand pane there is "Automatic Login" which you can set to your name. You may have to enter your password again.
Close "System Preferences" and you are done. Next time you restart your computer you will go straight into your account.