An operating method of an image sensor includes the following steps. The image sensor includes at least one pixel unit. The pixel unit includes a photoelectric conversion unit, a first control unit, a capacitor unit, and a sensing unit. The photoelectric conversion unit includes a quantum film photoelectric conversion unit, and the first control unit includes an oxide semiconductor transistor. The capacitor unit is coupled to the first control unit, and the sensing unit is configured to sense signals at a sense point coupled between the first control unit and the sensing unit. The pixel unit is discharged before a readout operation. The capacitor unit is charged by electrons emitted from the photoelectric conversion unit when the photoelectric conversion unit is excited by light. Signals at the sense point are then sensed by the sensing unit.