A method for smoothing a printed circuit board (PCB), comprising: providing a PCB having a first smooth outer surface and an opposite second outer surface, the second outer surface including a smooth region and a plurality of dimples; applying a liquid photoresist layer onto the second outer surface of the PCB to fill the dimples; solidifying the liquid photoresist in the dimples to obtain a solidified photoresist layer; polishing the solidified photoresist layer until a surface thereof being coplanar with the smooth region; and polishing the entire second outer surface until the solidified photoresist layer is removed, thereby obtaining a plain outer surface parallel to the first smooth outer surface.