Im not sure if thats a 3d editor thing. If it is, then youll have to import your model into a 3d editor like Blender and find the menu item set smooth. Its just a visual thing it does not change the mesh. The import and reexport will change the mesh.
I assume its changing the normals on the mesh to draw smooth.
Otherwise at runtime you could compute the normals but thats a function math thing.
thx，i has resolve this problem by this way：How to smooth an OBJ with threejs? - #13 by geomontgomery