The invention discloses a handheld tool and a clamping device thereof. The clamping device of the handheld tool comprises an installation seat, a clamping unit, an operation assembly and a stop mechanism, wherein the clamping unit is used for fixing a work head in the axial direction; the operation assembly comprises an operation part and a driving part, and the operation part is movably connected with the installation seat, thereby driving the driving part to be matched with and break away from the clamping unit selectively in an operationable mode; the stop mechanism is arranged between the installation seat and the operation part; and when the driving part is matched with the clamping unit, the stop mechanism restrains the driving part from breaking the matching with the clamping unit. The handheld tool is provided with the stop mechanism, guarantees smooth dismounting and mounting of the work head, and avoids the situation that in the dismounting and mounting process of the work head, the operation part moves further, the driving part breaks the matching with the clamping unit, and the work head can not be dismounted or mounted normally.