A charging circuit of a portable device includes a switch circuit having an input connected to a storage battery of the portable device for triggering the storage battery to supply power to an electronic device, a reference voltage circuit, a regulating circuit, a filter circuit filtering voltage signals from the storage battery, a collecting circuit, a comparator circuit, a rectangular generator, an integral circuit, and a driving circuit. The reference voltage circuit and the collecting circuit respectively output a reference voltage signal and a collecting voltage signal to the comparator circuit. The rectangular wave generator outputs a rectangular wave to the integral circuit. The integral circuit outputs a triangle wave signal to the driving circuit. The driving circuit outputs a pulse width modulation (PWM) signal to drive the regulating circuit to charge the electronic device.