Accurate calibration of the spectrometer based on incoherent broadband cavity enhanced absorption spectroscopy (IBBCEAS) is very essential for quantitative measurements of trace gases in the atmosphere. An incoherent broadband cavity enhanced absorption spectrometer using a blue light emitting diode as light source is developed for quantitative detection of atmospheric NO2. This paper describes three methods for calibrating the mirror reflectivity of the spectrometer in the spectral region of 430～490 nm, which are: (1) only using known absorption of standard NO2; (2) using known absorptions of standard NO2 and oxygen collisional pair in pure oxygen; (3) using Rayleigh extinction coefficient of pure nitrogen and helium. The wavelengths corresponding to maximum reflectivity calibrated with these methods are all about 460 nm, whereas different maximum reflectivity and respectively 0.999 25, 0.999 33 and 0.999 37. NO2 samples with various mixing ratios are measured with the IBBCEAS spectrometer in order to compare these three methods. Disagrees of about 14% and 19% between method (1) and other two methods are respectively found, whereas only about 4% disagree between method (2) and (3). The measuring results show that the concentration uncertainty of NO2 sample and inner wall loss maybe deteriorate the calibration accuracy of method (1) and hence this approach is not recommended. High sensitivity and calibration methods of the IBBCEAS spectrometer are further demonstrated by simultaneously measuring the absorption of atmospheric NO2 and oxygen collisional pair in the atmosphere in the spectral region of 440～485 nm.