When I want to delete or edit a sketch ,the model change or disappear in same time. It looks like a feature in beta version, but it happened in live version.
so may anyone tell me,how to disable this feature please 
You are using the history based version based on the screenshot that you shared. There is the history button on the right. It’s not possible to disable it. You can hide the sketches instead of deleting them.
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sir, I want to know what if we keep these sketches even the related model has been deleted. may it affect the render performance, i was quite worry about it sometime. would it be better to delete them too? 
It does not affect performance.
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Maybe it’s worth to think about “turn on/off” function for history (just like that done in Fusion 360). Not everyone appreciate this functionality as I do.
We won’t implement turning off history, but there will be an option to collapse the feature tree and get rid of the recorded steps.
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For me that’s more then enough. Will be very useful.
it’s bad news for me
can sketch disconnect body
Collapsing the feature tree will remove all dependencies.
I don’t understand that
delete sketch body will delete too?
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When dependencies will be removed then you’ll be able to delete sketches without deleting objects. As far as I understand.
I now have a “Junk” folder in each of my projects. The folder is hidden, and I drag all the sketches and planes into when I am done with them. Kind of annoying, but it works.