Incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) has a very high detection sensitivity, nevertheless the quantitative results retrieved by the traditional concentration retrieval method are affected by the calibration error of mirror reflectivity. Therefore, in this paper we present another concentration retrieval method based on the measurements of atmospheric O2-O2 absorption. In this method the optical cavity of IBBCEAS is equivalent to a multi-reflection cell, in which the optical path length is independent of the wavelength, i.e. a constant. First, we get the slanting column concentrations of atmospheric O2-O2 and other trace gases measured by using the differential optical absorption spectroscopy (DOAS) to fit the corrected cross sections of the measured gases to the optical density from the IBBCEAS absorption spectra and reference spectra. Second, the optical path length of an equivalent cell is determined by the known concentrations of O2-O2 in the atmosphere. Third, the concentrations measured for trace gases are retrieved by the deduction of absorption optical path length from the obtained slanting column concentrations. The above method is demonstrated by measuring the atmospheric NO2 with an IBBCEAS instrument in the range of 454-487 nm. Atmospheric NO2 concentrations retrieved by this method are compared with those by the traditional method, and the difference between them is shown to be less than 7%. Experimental results show that the absorption of atmospheric O2-O2 can be used to quantify other trace gases when measured by IBBCEAS, and, above all, the quantitative results are almost insensitive to the calibration error from mirror reflectivity.