An illuminating device which accesses natural energy is an independent cylindrical unit, an interior is provided with a once-through passage to absorb solar thermal radiation wave which then operates on air molecules inside the once-through passage to increase an ionization speed, so as to form resultant force. The resultant force is then converted into mechanical energy which operates a suspended air screw to rotate, so as to drive a magneto generator to convert into an electric current which is used for illuminating through an access control.