The invention provides a gas detection device and method, which can be used for real-time online tracking and monitoring surface reaction under high vacuum, high temperature and high pressure condition and performing qualitative quantitative accurate measurement and are combined with in-situ infrared reaction tank-mass spectrometer. According to the device and method, an in-situ reaction tank anda spectrometer are organically related through a six-way vacuum cavity, functions of separate use of spectrometer, separate use of an in-situ reaction tank and combined use of reaction tank-mass spectrometer can be achieved through gas flow path online switching under the situation that the high-pressure high-vacuum condition of a reaction system is ensured and normal performance of a surface reaction is not influenced, qualitative and quantitative detection of gas products under high vacuum can be realized, the change process of reactants, intermediate products and final products can be accurately detected online in real time, qualitative and quantitative analysis of the reaction can be realized, and accurate description of the surface reaction mechanism can be effectively improved.