An air pump has a bracket, a motor, a transmission device and pneumatic cylinder assembly. The bracket is formed from sheet metal and has a backplate, two side plates formed on the backplate and two top plates formed respectively on the side plates. The motor is mounted on the transmission device. The transmission device is mounted on the bracket and is connected to the motor. The pneumatic cylinder assembly is mounted on the bracket and is connected to the transmission device. The bracket formed from sheet metal is light and has a low cost.