Thank you for your answers, guys !

I had just tested and found that the keyboard back light works as you said.

Actually, I use a transparent screen protector to protect the screen from scrach, but the light sensor is covered by this protector.

It makes the sensor not that sensitive.

So everytime I open the keyboard, the back light of the keyboard always turned on.

To figure it out, then I used a flashlight to focus the light on the sensor and at the same time, opening the keyboard. This time the back light does not turned on, then I know that what you guys said it is just right.

Thank you again !