The invention provides a gas leakage detecting device and a detecting method thereof. The gas leakage detecting device and the detecting method thereof are capable of performing high-efficiency and high-sensitivity detection on gas leakage of an airtight vessel at high altitude and are applicable to rapid detection of airtightness and rapid positioning of a large flexible structural vessel. The device is characterized in that a creeping and positioning assembly is used for carrying a mass spectrometer; a single leaking point of an airship is quantified and positioned at high altitude by adopting a tracer gas method, so that the detection can be quickly and efficiently carried out; in addition, a vacuum pump is used for rapidly pumping gas, thus gas in a certain volume can be concentrated and stored in a vacuum chamber within the shortest time, and as a result, the gas collecting capacity is improved; the mass spectrometer is used for sampling and detecting concentrated and mixed gas inthe vacuum chamber by utilizing a circulating fan, and thus the equipment detection efficiency is improved. According to the device, the gas is collected by adopting an injection and gas collecting unit; compared with a general gas collecting device, an injection hole is added, so that the detection sensitivity is improved; a chuck body is used for adsorbing a detected target under negative pressure as well as absorbing gas.