A photo sensing unit used in a photo sensor includes a photo sensing transistor, a storage capacitor, and a switching transistor. The photo sensing transistor receives a light signal for inducing a photo current correspondingly, and a source and a gate thereof are respectively coupled to the first signal source and the second signal source. The storage capacitor stores electrical charges induced by the light signal, one terminal thereof is coupled to drain of the photo sensing transistor, and another terminal thereof is coupled to a low voltage. The switching transistor is controlled by the second signal source for outputting a readout signal from the storage capacitor to the signal readout line. The threshold voltage of the photo transistor is higher than that of the switching transistor.