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I really searched a lot of files, but I can't find about that....
Is there any API to auto lock screen, turn off screen, but not turn off power?
No, these API are not currently available. Here's the same topic:
https://social.msdn.microsoft.com/Forums/en-US/a3df5079-a549-4b74-8dad-24d82b3173e8/windows-phone-81-lock-screen-api?forum=wpdevelop
--Simon