The invention discloses a lithium-ion battery pack thermal runway automatic alarming apparatus based on gas monitoring and a monitoring method, and relates to a thermal runway alarming device for letting out flammable and combustible gas for real-time monitoring and timely alarming during battery thermal runway aiming at the warehousing and charging process of a lithium-ion battery pack. The lithium-ion battery pack thermal runway automatic alarming apparatus is composed of a collecting device, a gas monitoring device, a controlling device and an alarming device, wherein the collecting deviceis composed of a collecting hood, a back-fire relief valve and a gas pump; the collecting hood is installed above a positive electrode of the lithium-ion battery pack, and is connected with a gas inlet of the gas pump through a gas pipeline, and the gas pipeline is internally provided with the back-fire relief valve; the gas monitoring device is composed of a gas collector, a gas sensor and a Dupont line. A gas outlet of the gas pump is connected with the gas collecting box, and the gas collecting box is internally provided with the gas sensor; the controlling device is composed of a control box, a single-chip microcomputer, a gas sensor, a gas sensor interface and a control electric circuit; the alarming device is composed of a blue LED alarming lamp, a red LED alarming lamp and a buzzer.