An air supply device, comprising a motor, a reducer connected to a spindle of the motor, a driving shaft connected to an output shaft of the reducer, and a connecting rod connected to the driving shaft; a cylinder having a piston disposed therein, the connecting rod being sealed inside the cylinder and connected to the piston; a control circuit, the motor being connected to an output end of the control circuit through wires; a storage battery connected to the motor and the control circuit through the wires to supply power thereto; the piston including an adjusting lever penetrated therethrough and being movable with respect to the piston, the adjusting lever having an air supplement hole; the air supply device having advantage of small size, light weight, convenience in use, and power saving.