A readout circuit for reading sensed signals of a pixel array. The readout circuit includes: an operational amplifier, a plurality of switching devices and a computation circuit. The operational amplifier is arranged to generate an output signal in each amplifier output cycle. In each amplifier output cycle, each of the switching devices is controlled by a switch controlling signal to selectively couple pixel circuits to the differential input terminals of the operational amplifier. The computation circuit is arranged to the recover a plurality of sensed signal respectively corresponding to a plurality of pixel circuits. In each amplifier output cycle, at least two of the switching devices are turned on, such that the operational amplifier receives the sensed signals of at least two pixel circuits in the pixel array simultaneously and sums the sensed signals of the at least two pixel circuits to generate the output signal.