The invention discloses a device and a method for measuring the concentration of carbon dioxide in sulfur hexafluoride decomposition gas and relates to the technical field of photoelectric technologydetection. The invention aims at solving the problems of low corresponding speed and low measurement precision of an existing device and an existing method for measuring the concentration of CO2 in the SF6 decomposition gas. A 1910 cm<-1> laser is controlled to emit laser with the wave band of 1910 cm<-1>; the laser is transmited in a sample cell through a first converging lens; the laser with thewave band of 1910 cm<-1> is used for absorbing CO2 gas in the SF6 decomposition gas in the sample cell, then is transmitted to a second converging lens and is subsequently transmitted into an entrance slit of a spectrograph; a spectrum of the CO2 gas is obtained by the spectrograph; the spectrum of the CO2 gas and an atmospheric spectrum are obtained by storage of a digital lock-in amplifier; andthe concentration of the CO2 gas charged into the sample cell is obtained by a computer. The device and the method are used for measuring the concentration of the CO2 in the SF6 decomposition gas.