A gas storage device includes a hollow casing, a gas storage bag, a gas tube, and a retractable module. The hollow casing has a casing top, a casing sidewall, and a casing bottom. The gas storage bag is located in the hollow casing. The gas tube is arranged through the casing bottom and in fluid communication between the gas storage bag and an exterior space. The retractable module is connected between a top surface of the gas storage bag and the casing top. The gas storage bag expands when fluid is filled into the gas storage bag through the gas tube, such that the retractable module is folded. The gas storage bag contracts when fluid is drawn out of the gas storage bag through the gas tube, such that the retractable module is unfolded.