A pneumatic ratchet wrench includes a main body formed with a receiving chamber, a head unit, a driving unit and an air motor. The head unit defines a gas passage, and includes a ring portion and a frame portion. The ring portion includes an exit in spatial communication with the gas passage and opens toward the frame portion. The driving unit includes a driving head, and a transmission shaft mounted in the head unit. The air motor drives the transmission shaft and includes a cylinder and a rotor. The cylinder includes a venting hole allowing gas produced in the cylinder to pass through the venting hole and the gas passage and to exit from the exit to be blown toward the frame portion.