A charging device for providing an output voltage to charge a flash capacitor is provided. A transformer includes a primary winding and a secondary winding. The transformer generates a primary voltage according to an input voltage and generates a secondary voltage according to the primary voltage. A diode coupled between the secondary winding of the transformer and the flash capacitor provides the output voltage according to the secondary voltage. A current detector detects a current flowing through the primary winding of the transformer and generates a detection signal. A determining circuit generates a determining signal according to the primary voltage, the secondary voltage and a reference voltage. A control circuit switches a switch coupled between the primary winding of the transformer and a ground according to the detection signal and the determining signal, so as to control the transformer to charge the flash capacitor.