Thanks for looking into it. I've uploaded a pic. The left one is with shadow strength 1 and the right one is 0 strength.
It's hard to see the shadow cast from the skeleton animation object(the winged girl) because for some reason the shadow is very long and scattered as if it has a dissolve image effect applied. It doesn't happen when with directional light though.
When I use transparent/cut out/diffuse shader, the shadow looks ok.
It's been solved. You can just use Fallback and let "Transparent/Cutout/Diffuse" or other shader to handle the shadow part.
