A gas sensing device includes a substrate, a conductive unit, and a sensing layer. The conductive unit is disposed on the substrate, and includes two electrodes. The sensing layer is disposed on the conductive unit, and is electrically connected with the electrodes. The sensing layer adapted to absorb carbon dioxide includes polyethyleneimine and polyethylene glycol. A detecting system including a testing device, an analyzing device and the aforementioned gas sensing device is also disclosed. The gas sensing device is detachably mounted to and is electrically connected to the testing device. Electrical property of the gas sensing device 100 changes when the gas sensing device 100 absorbs carbon dioxide.