The invention discloses an internally clamped clamping mechanism for a barrel. The internally clamped clamping mechanism for the barrel is arranged on a working platform, is used for clamping a metal barrel and comprises a first clamping block and a second clamping block which are mutually matched, wherein the first claming block is arranged on the working platform along the vertical direction; and the second clamping block is provided with a contraction device and an expansion device along the horizontal direction relative to the arrangement of the first clamping block; the second clamping block slides along the direction close to the first clamping block to lead the second clamping block to be in a contraction position; the metal barrel is sheathed on the first clamping block and the second clamping block; the second clamping block slides along the direction far away from the first clamping block to lead the second clamping block to be in an expansion position; and the inner wall of the metal barrel sheathed on the first clamping block and the second clamping block are commonly clamped by the first clamping block and the second clamping block. With the adoption of the internally clamped clamping mechanism for the barrel, the scrapping of the metal barrel caused by deformation of the metal barrel due to the fact that the metal barrel is externally clamped can be effectively prevented.