Abstract: An air pump consisting of a housing, a piston, a piston-rod, and a cover is disclosed. The housing is a open-top hollow cylinder which contains a lower portion of the piston-rod and the piston connected to the bottom end of the piston-rod. The housing is topped by the cover which has a circular hole located at the center so as to allow the piston-rod to pass through the cover into the housing. The piston is connected with the piston-rod in a manner that the piston can rotate around the connecting point to some small degree while remains connected with the piston-rod. This may prevent the deviation of the piston from the position perpendicular to the inner wall of the cylinder caused by the sway of the piston-rod when the piston-rod is moved up and down. In addition, a cavity is formed in the surface of the piston-rod near the top to retain a ball needle.