A CMOS image sensor, a pixel unit and a control method thereof are provided. The pixel unit includes: a photoelectric conversion unit, an isolation transistor, a storage unit and a reading unit, wherein a first terminal of the isolation transistor is connected to the photoelectric conversion unit, a second terminal of the isolation transistor is connected to the storage unit and the reading unit; and wherein the storage unit comprises a first switch unit, a second switch unit, a first storage capacitor, a second storage capacitor and a reset unit, the first switch unit is adapted for controlling the first storage capacitor to be charged or discharged, the second switch unit is connected to the second storage capacitor and is adapted for controlling the second storage capacitor to be charged or discharged. The pixel unit improves signal-to-noise ratio of image signals generated by the pixel unit.