The invention relates to an apparatus for measuring carbon dioxide concentration in sulfur hexafluoride decomposition gas and a method for measuring carbon dioxide concentration based on the apparatus thereof, which belong to the field of photoelectric technical detection. The apparatus and the method solve the problems that the current carbon dioxide gas concentration method cannot simultaneously satisfy high measurement precision and low cost of the measurement equipment, and real-time monitoring cannot be realized. The apparatus for measuring carbon dioxide concentration comprises a laser, a first collecting lens, a sample tank, a second collecting lens and a spectrometer, laser emitted by the laser is transmitted to the sample tank through the first collecting lens, the sample tank transmits the laser to the second collecting lens, the laser is transmitted by the second collecting lens to a incident slit of the spectrometer; and the incident slit of the spectrometer is positioned at a focus position of the second collecting lens. the sample tank is filled with atmosphere, the laser with 4841 cm<-1> is opened, and the atmosphere spectrum can be obtained by the spectrometer; the sample tank is filled with carbon dioxide, carbon dioxide spectrum is obtained by the spectrometer; and the carbon dioxide concentration can be obtained by Beer's law. The apparatus and the method are used for detecting the sulfur hexafluoride decomposition gas.