What's wrong with it? How can I solve it?Thanks a lot for your advice.
Here is a link that might be useful:
It's a little hard to tell from your picture, but it looks like Miscanthus Blight, caused by the fungus Leptosphaeria sp. and its imperfect form Stagonospora spp..
Prune off and destroy affected leaves as soon as the spots appear.
Spray the remaining foliage with an organic copper-based fungicide. Follow label directions carefully.
In late fall, rake up and dispose of any withered leaves and other plant debris.