An air pump has a body, a piston rod and a seat cap assembly. The body has an outer protrusion and a first engaging part. The outer protrusion protrudes from the body. The first engaging part is longitudinally mounted on the outer protrusion. The piston rod is inserted into the body. The seal cap assembly has a top cover and a sleeve. The top cover is movably mounted on the piston rod. The sleeve is mounted on the body and has a retaining rib and a second engaging part. The retaining rib protrudes from an inner surface of the sleeve and abuts against a retaining surface of the outer protrusion. The second engaging part is longitudinally mounted on the inner surface of the sleeve and is engaged with the first engaging part of the body. Therefore, the relative rotation between the body and the sleeve is prevented.