Cavity ring down spectroscopy (CRDS) not only has high sensitivity, but also can measure the absolute absorption of the sample gas. The decay time of the oxygen molecule at 17266.090 cm-1 is recorded at system's limiting vacuum and other different pressure of oxygen filled in the cavity. Therefore, the lifetime of the empty cavity is obtained to be 2.9174 ms by approaching method, and the absolute absorption cross section of RQ(5) ro-vibrational line of the (3, 0) band in the triple forbidden transition b1∑+g–X3∑-g of O2 is determined to be 1.4998(±0.0967)×10-26 cm2, which is in good agreement with previous estimation, by employing continuous wave CRDS. Additionally, the reflectance of the two identical cavity mirrors, obtained by the approached empty cavity lifetime, is determined to be 99.989 (±0.001)%, which is more accurate than usual method, and the equivalent absorption length is prolonged by a factor of about 9090 in our experimental condition.