Photoacoustic spectroscopy is a kind of high sensitive detection method. Cell constant is an important parameter of photoacoustic detection system, which directly measurements the system′s ability of converting light energy into sound energy. So the calibration of the cell constant is an important job for photoacoustic detection system′s metrology. Oxygen in the air is used to calibrate the cell constant. For a first order resonant cell, of which the length is 100 mm, and the diameter is 6 mm, the result of the calibration is 2330.8 Pa·cm·W-1, and the repeatability is 0.67%. Compared to the standard gas calibration result, the relative error is 0.54%. Experiments prove that it′s feasible to calibrate the cell constant online using oxygen in the air.