The invention discloses method for measuring the concentration of CF4 gas in a SF6 decomposer by utilizing a spectral absorption method, belongs to the technical field of photoelectric detection, and aims to solve the problems that high measurement accuracy and low cost of measurement equipment cannot be simultaneously met and the on-line monitoring cannot be realized in a conventional CF4 gas concentration measurement method. The method for measuring the concentration of the CF4 gas specifically comprises the following steps: introducing the atmosphere into a sample cell, changing laser beams emitted by a laser into parallel beams through a first collecting lens so as to come into the sample cell, converging the transmitted light beams into an entrance slit of a spectrograph by a second collecting lens, and acquiring a spectrum of the atmosphere in the sample cell through the second collecting lens; replacing the atmosphere in the sample cell by gas CF4 to be detected; repeatedly performing the optical signal processing process, and acquiring a spectrum of the gas CF4 to be detected; and acquiring the concentration of the gas CF4 to be detected according to a beer's law. The method is used in a SF6 detector.