The study proposes a new readout circuit for an organic gas sensor. The readout circuit consists of a preamplifier, a sample and hold circuit and a differential current amplifier. A preamplifier composed of trans-impedance to convert the output current of a gas sensor into voltage. The sample and hold circuit memorizes various voltages according to distinct designed time instants in a target gas environment. At the same time it is equipped with the advantage of repeated detections by the controlling signals which determine the frequency of switch. The differential current is amplified by a differential current amplifier and is attributable to the voltages of sample and hold circuits using current mirror framework. Flowing through the resistance load, the differential current is converted into output voltage. A microprocessor control unit (MCU) acquires and stores the outputs in order to determine gas concentration database of pre-measured which are contrast. The designed circuit is accomplished by Taiwan Semiconductor Manufacturing Company (TSMC) 0.35μm 2P4M 3.3V mixed-signal CMOS process.