When I am drawing on the mesh surface ，the offset changes if you draw at distance， that is odd.
Hey. This is a known limitation with an offset > 0. It’s because of …math… I guess 
Depending on what you’re aiming for you could either draw in orthographic view or set the offset to zero and maybe add a shrink wrap modifier on the GP Object with outside or above surface if the mesh “swallows” parts of your stroke
Thank you, it helped in viewport, but in the final render, the mesh swallows the stroke
I confirmed that I rendered in object mode and have applied modifiers
hmmm 
strange. could you upload the file?
I think it’s just because the sphere is really large, like ~50 meters. So you have to crank up the offset in the modifier a little more. No idea why the viewport and render looked different in your original file though…
OK… I try to explore it myself. 