About the sprint timeline

but sometimes PO change the sprint length to 1 week or shorter according to the spring backlogs

What does the Scrum Guide say about sprint lengths? And changes to it?

yes it says PO can make changes to sprint but not sure about the time length of sprint.

The Scrum Guide states: Sprints have consistent durations throughout a development effort

And: Once a Sprint begins, its duration is fixed and cannot be shortened or lengthened.

Sprint length can be changed during retrospective by the team, and should ONLY be done if emperical evidence showed a chnage in sprint length is necessary, and the team is aligned on this as a whole.

Factors from evidence concerning change are things like market circumstances, risk, alignment/synchronization within the rest of the development organiziation.

Changes to the sprint length have (possibly negative) consequences on things like cadance, predictability, velocity and forecasting.

So, that said, how would you feel about your inital question?

Just to give you a heads up, if you have no thoughts on it yet...

First of all, there is no such thing as "The PO chnages..." in general a single person deciding is against the team and sel organizing principles of Scrum. So if there are compelling reasons to change the sprint length, the PO should collaberate with the team about this and a collective descicion should be made on this. There is only the team as a whole here.

Next to this, in this discussion, the negative factors should be discussed as well and taken into account.

Back to your question, I want to give you the first thing that crossed my mind...

Not having a Sprint Backlog which accommodates a "normal" sprint is a problem. It is something that should be addressed and resolved. Changing the sprint length is fighting a symptom, but not curing the underlying problem.

Actually, it is covering it up and sweeping it under the rug.

So instead of making PO change the sprint length, try to find out why there is no suitable sprint backlog.

It can be multiple things (and a combination of mulitple things), like a bad PO, not enough refinement time, unclear product backlog or company vision on this, problems with stakeholders, and a milion other things.

As a SM (and team in general) you should put in max effort in making the problems transparent!

Next to that, if it about releasing as fast as possible (dont know if it is) keep in mind that the team can release at any point in time. Releases are not bound to Sprint length (except each sprint should at least have a potentially releasable Done increment at the end). So no sprint length chnage is needed to release more often, i the ony thing I would like to point out.

So, good look inspecting the real cause of the request to change the sprint length, and adapting to solving this cause!

yes it says PO can make changes to sprint

Are you sure? Wouldn't any such changes have to be negotiated?