Hi, when people's stack was short, usually will use an all-in or fold strategy（Especially during the tournament).

I want use SPR to represent，because some game with ante or straddle, on a regular game 20BB equal SPR13 (20/1.5) on the preflop.

my question is what's the threshold of this strategy? when the SPR drop to which number we can enter an all-in or fold since this strategy will +ev than limp or open?

and when we face open, which SPR we can use 3bet all-in to replace 3bet 2x—4x?

SPR is used on the flop, not pre

According to who?

The people who invented the term?

SPR can be used for any street (it's literally the ratio between the size of the effective stack and the size of the current pot), but it's most commonly used on the flop, to give you an idea of whether you should be committing to your hand (i.e. getting all in at some point).

If the effective stack pre-flop is 20bb or less, then you don't really need to do any calculations. While you might occasionally 3-bet/fold, you'll usually either shove (or 3-bet shove) pre, or raise or call pre-flop and then make your decision on the flop. (Where the SPR will often be <3). I suppose if you are in the BB facing a raise pre, you could quickly add up the money in the pot and compare it to the size of your stack, but most tournament players already have a "feel" for the best play in that spot. With some hands they fold, some they 3-bet jam pre, and with others they call, and then either donkbet or go for a check-jam. Occasionally, you're still deep enough on the flop to check-call the flop and then stack off on the turn, or even check-call flop, check-fold turn. Generally speaking, however, if you start the hand with <20bb, you'll either be getting it in pre-flop or on the flop. With stacks of 30bb or more, you've got plenty of stack depth for some post-flop play. Just watch the early levels of a SpinGo, or the final table of an MTT. It's not all just shove or fold. You're only really limited to those two options when you have less than 10bb.

P.S. For working out your pre-flop shoving ranges, you might want to use the M number (which is derived from the blinds and antes you need to pay each orbit) for building your shoving ranges, as popularized by Dan Harrington, instead of solely using big blinds.

Isn't M (unadjusted for number of players at the table) the same as SPR?

As Arty said, M is generate by adding up antes and blinds to calculate how many hands you have left before blinding off.

SPR is stack-to-pot ratio for this particular hand

I was thinking if you are first to act in an unopened pot. If someone before you calls or raises, it changes everything.

It's basically the same idea (except that the moment someone opens the pot the SPR changes, but your M doesn't) and there are already various charts giving advice on shoving/re-shoving ranges based on M.

They didn't invent the term. They were the first to popularize it's use, and maybe the first to use it. I could say "stack-to-pot ratio" and anyone with a basic understanding of the English language and poker will know what it means. The people you're referring to focused it's use on the flop stack-to-pot ratio, but there's no reason it can't or shouldn't be used more generally.

That's fair enough, but as the first to really popularise it in terms of using it to make decisions at the key point in the hand (ie. the flop), that's when it should be used AFAIC

Paul Magriel* was the one that first called it M. It was popularized in Dan Harringtion's book, in which he credits Magriel for deciding what to call the concept.

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*https://en.wikipedia.org/wiki/Paul_Magriel