Shadow flickering for large shadow distance

I need 10000 shadow distance for the environment objects. However, at this shadow distance, my character’s shadow keeps flickering and shaking.
Did I do something wrong? Or how do I remove the flickering?

A 10,000 metre shadow distance is just not viable for realtime shadowmaps - at that scale you would have over a metre for a single pixel in a best-case scenario (without cascades). If you want large scale shadows, you will at the very least need 4 shadow cascades (with the near cascades set way down) and even then it would be a better idea to bake shadows and use distance shadowmasking than try and cover everything with shadowmaps alone.