The function of the 'Offset' button is abnormal.
WHY?
What is abnormal? It is expected that offset changes the curves in the Curves view. Look at the curves in the Graph view to better understand what is happening.
什么是不正常？预计偏移会改变 "曲线 "视图中的曲线。看一下 "图表 "视图中的曲线，可以更好地了解发生了什么。
The Curves view is very hard to see the actual shape of the curve. If you use 4.0+ you can see the curve in the Graph view and that should make it clear why the curve is being changed.
Reinstalling won't change how this works, sorry. You can adjust your curves as normal after offset.
Assuming there isn't a bug in 3.8.99, and I don't think there is, when you offset Spine "splits" the curves to keep the same interpolation. The 3.8 graph makes it difficult to see what is going on, so the curves look strange, but they are correct. Make a simple animation that moves a bone, set curves, then offset. You'll see the motion stays the same, even if they curves look strange.
I sovle this problem after reinstalling.The curve is correct now.maybe u didnt get my point.The second curve means,the curve after the second Rotation Key.After i offset,I shoud be a beautiful curve,not like a'√'.I dont know if there is an button could let me adjust 'how the way offset goes'.Maybe I pressed the wrong one。
still many thanks Nate,have a nice day
I don't know why a reinstall would help, but I'm very glad it's working like you want! 
I also encountered the same situation. (spine ver.3.8.99)
The reason is probably that when you use offset, the curve of the second key is not the 0F curve, but the final F curve.
And in spine3.xx and below, the final F key cannot be edited.
The workaround is to copy the 0F key with the adjusted curve to the final F before using offset.