In TES:IV if an actor's fatigue goes down below zero,the actor will be forced to play a knock down untill the fatigue recover to above zero,yet in TES:V I try it but never see such thing, then I checked a function called GetKnockedState in Papypus, it shows that the return value "3) negative fatigue knockout" still exists,anybody knows the way to make it?

Doesn't happen...if you don't have the fatigue to do something, you simply can't do it...the bar flashes and that's it.

If you can find an example of GetKnockedState being used in the vanilla game/scripts, look at that. If you can't find a working example...my guess is it doesn't work.

I'd like to know that as well, but can't find any use of it in the game. Ulfric and Tullius get knocked down at the end of the Civil War questline, but I can't find their script ...

that seems like the normol follower's behavior,get knocked down when health is low,although I can't simply find any script that makes one get knocked down.

Guessing they just have their health regeneration turned off so they stay in the "Essential NPC wait to heal" mode and can't get back up. Probably not related to fatigue at all.