The invention relates to the field of measurement devices, and discloses a locking assembly of a pneumatic measurement device. The locking assembly comprises a sliding arm, wherein the sliding arm isprovided with a bracket, an air cylinder is arranged on the bracket, a push-pull rod is arranged in the air cylinder, the push-pull rod is provided with a connecting shaft, the sliding arm is providedwith a rotating shaft, the rotating shaft is provided with a rotating arm, the rotating arm is provided with a measuring head, one end of a transmission chain is connected with the sliding arm, and the other end of the transmission chain is connected with the rotating shaft, the transmission chain is bent to one side of the air cylinder, a transmission rod is arranged at the bending position, theconnecting shaft is connected with the transmission rod, a band-type brake is arranged outside the connecting shaft, a holding rod is fixedly connected with the band-type brake, and the holding rod is fixed to the sliding arm; the sliding arm is provided with a supporting block, the end, making contact with the rotating shaft, of the supporting block is connected with an air bag, and the air bagis filled with reserved gas; and in the self-locking process, the resistance between the top end of the holding rod and the connecting shaft is linearly increased until the locking is completed, and the air bag enables the use of the measuring device to be more stable.