This obj file is always black

Ladies and gentlemen, I have a problem. This obj file is always black.
The sketchfab test doesn’t have this problem.

Whether I change the material type or change it to double-sided, it is still black.

Add normals to your geometry.

Thanks the reply,Solved the problem.
I used compute vertex normals.But the smoothness of the model is gone.

Because of how the OBJ format is structured it’s more or less loaded as a soup of triangles in three.js. If you want to generate smooth normals for the model you’ll want to merge the vertices of the model generating them using something like BufferGeometryUtils.mergeVertices which is probably similar to what SketchFab is doing. Even then, though, this can get complicated if there are some edges that you want to keep sharp. But generally I’d recommend a different file format and including the normals in the original file to make sure the model looks the way you want.

Those are called seams. They are the result of vertices occupying the same space but having different normal values. Doing normals is not an exact science and user input is required.

The best way to smooth out these seams is to make sure vertices that share the same coords, share same normal value.
Human input is required to determine what vertices should be neutralized.