An improved air compressor includes a cylinder with a piston body and an air storage container. The air storage container can be detachably mounted to the cylinder to define a primary air chamber, and an auxiliary air chamber which can reduce the motion resistance of the piston body, so that the piston body can conduct reciprocating motion in the cylinder more smoothly. Furthermore, the cylinder has an open bottom that is divided into two halves according to a central vertical line (Y) of the cylinder, wherein one half of the open bottom is horizontal while the other half of the open bottom is slanted. When the piston body reaches the bottom dead center, the head of the piston body will be entirely within the cylinder and thus keep air-tight with the cylinder. Therefore, the performance of compressing air and the operational safety can be increased.