An inflator includes a hollow handgrip and an air cylinder having a shaft. One end of the shaft is connected with a piston in the cylinder and the other end of the shaft is provided with a stud extending out of the cylinder. A bottom wall and an opening are arranged respectively at opposite ends of the handgrip. A tapped hole is formed in the bottom wall for locking the stud. Through the opening, the cylinder may be stored in the handgrip to lessen the volume of the inflator. When using the inflator, a user draws the cylinder out of the handgrip and lock the stud to the tapped hole through the bottom end of the handgrip to thereby extend the shaft. Air is pumped out through an outlet of the cylinder by reciprocal axial motion of the shaft.