Wlachan, you are very right, that's what I just found.
As show in this sketch, the blue part is the AF driver, and the purple one is the focusing ring with distance marks, they are fixed together by the black screws.
To adjust the distance marks, the screws holes on the focusing ring is made as pierced arc, so when the screws loosen, the two part will move a little bit and then can't focusing infinite.
But why infinite? I guess when the AF driver push the lens move toward infinite, there will be a sudden stop when the focusing ring hit the stop block, again and agin, the lens may be rush a little over the infinite, but the focusing ring can't go over the block, then problem come.
And, there are some photo I took when disassembling it.
This is the lens before disassembling.
Front cover Removed, they are sticked there by double side adhesive tape. The three screws there is used to fix the lens cover.
remove all the cover, then showed the three screws fixed the focusing ring, and the arc.
Its not necessary to withdraw the screw to adjust , loosen them is enough
the two part are connected only by the three screws, as they together turn and stop so often, I think loosen is inescapable.
the AF driver part.
After adjusting, I go out to find a distant thing, in midnight, moon is best.
with aperture wide open, 100% crop.
well, it seems all right, the only problem is that even moon is not at infinite marks, it's between the 1.3 and infinite. I have adjusted too much, but there will be no 'ka' noise and collision of focusing ring and the block any more.
I will test the lens more tomorrow.
Thank you all.