Thank you for your prompt reply!
I think this mistake should be caused by what you said.
However, I did the tests according to your steps and found that I still couldn't solve the problem. According to your operation steps, after using the "import Normals and Tangents" option in UE4, it is still hard edge display in the editor of UE4.
So I went on to do another test of importing FBX format, as follows:
1) first export my "boy_setup" to FBX format.
2) import the FBX format files into UE4, and select the "import Normals and Tangents" in the import option:
3) SoftEdge is displayed in "skeletal mesh editor".
What on earth is this? I want to ask if there is a problem with the default setting of my own UE4.18.3?