If I set the line art, I can see it well at the set angle. If I change the angle, the line art won't be updated. You can only see the old lines like the picture. How can I see the line art line in real time?
Since the Line Art modifier relies on the view frustum of the current scene's active camera, you can Lock the camera to view. (Press on N or click there to expand the right hand side panel)
I suggest you create a new camera for this purpose if you already have a camera setup you don't want to mess up since you can't undo viewport movements. In the Line Art modifier, set the camera to Custom and select it. (Only available in 3.0+, but you can simply create a new camera and temporarily set it as the active camera in the scene properties)
You should also increase the Overscan parameter of the Line Art modifier in order to draw the lines over the camera limits.
If you don't want the border to be dimmed you can disable the passepartout in the camera settings.
Go into Camera view mode with Numpad 0 or click on the camera icon in the viewport :
The Line Art modifier should update when you move the viewport camera :