A pressure relief device for an air inflator includes a pipe connector, a seal member, a check valve mounting ring, a check valve, an air cell connector, and a pressure relief control ring. Thus, the pressure relief control ring is tightened to perform the air inflating action and loosened to perform the pressure relief action, so that the air inflating action and the pressure relief action are performed along the same axis, and the pressure relief device has a co-axial operation function, thereby facilitating the user inflating and deflating the inflatable body. In addition, the air inlet hole is not mounted in the air cell to facilitate the user compressing the air cell.