A clamping jig for fixing a workpiece in position includes a base, a cover mounted on the base, a driving assembly and a clamping assembly. The cover has a top face for receiving the workpiece thereon. The driving assembly includes a diaphragm type pneumatic actuator fixed to the base and a linkage slideably received in the base for receiving a pushing force from the diaphragm type pneumatic actuator. The clamping assembly includes a plurality of clamping members each including a wedge slideably received in the base and an abutment extending upwardly from the wedge through the cover. Under action of the linkage to the wedges, the abutments are movable between a first position to engage with the workpiece to a second position to release their engagements from the workpiece.