A clamping mechanism for a circular saw machine contains a base; a support holder including two parallel sliding rails; an inner clamp device including a fixing mount, a first horizontal clamping module, and a first upper clamping module; an upper clamping module including a first positioning seat, a first affixing protrusion, and an auxiliary pressing projection; an outer clamp device including a moving pedestal, a second horizontal clamping module, and a second upper clamping module, the moving pedestal having an external clamp arm and a second clamp block, the second upper clamping module having a second movable fixing block; the second upper clamping module including a second positioning seat and a second affixing protrusion; a driving device including a servo motor and a drive column. The fixing mount has an internal clamp arm and a first clamp block, the first horizontal clamping module has a first movable fixing block.