The invention specifically relates to a high-sensitivity gas detection device and a method. The invention aims to solve the technical problems of easiness in being influenced by environment, low stability, low detection sensitivity and cross influence between different gases of the traditional SF6 decomposer detection method. According to the invention, the noise of a sensing system is effectively reduced in the manner of smartly designing a photo-acoustic cell with a difference structure. Besides, a multichannel time division multiplexing technique is adopted for realizing synchronous detection for multichannel gas in each photo-acoustic cell and accurately measuring the concentration of SF6 decomposer gas. Compared with a sensing system adopting a wide spectrum infrared source, the high-sensitivity gas detection device has the advantages of low cost, high detection sensitivity, high selectivity and capability of meeting the practical detection requirement for the SF6 decomposer in an electrical power system at present.