Why my 6s plus always consume 1.3 Gb of RAM even i emptied multitasking? How to cut off?

The operating system (iOS) takes about that much space.

And?

Are you experiencing issues?

Modern operating systems dynamically manage RAM.

They will anticipate the way you use your device and preload apps.

As the RAM is needed, the stuff not in use gets dropped.

Unless you are experiencing issues, there is no reason to worry about or look at the RAM usage-especially if you do not understand how the device works.

Damian234 wrote:

So how could the iPhone 6 works with only 1 Gb of RAM, even the 4s with 0.5 Gb.

The Operating System on your phone is far smarter than the Apps that claim to "free memory". Any such App is a waste of your time and the phone's resources.