A clamping mechanism automatically adaptable to change of thickness of printed circuit board includes: a frame body having two parallel rail seats; two clamping rail members respectively mounted on the rail seats for clamping a printed circuit board, each clamping rail member including a first clamping rail and a second clamping rail spaced from each other by a clamping gap; and two adjustment units for adjusting the clamping gap of the clamping rail members. Each the adjustment unit includes: a support section mounted on the frame body to provide a support face; a connection section, one end of the connection section being affixed to the first clamping rail, the other end of the connection section being positioned above the support face; and a floating section positioned between the connection section and the support section and supported on the support face to apply a resilient support force to the connection section.