The quantitative method of iodine monoxide radical (IO) using incoherent broadband cavity enhanced absorption spectroscopy (IBBCEAS) in the 435–465 nm band is described in this paper. In order to obtain the concentration of IO accurately, the parameters such as the mirror reflectivity, effective cavity length and sample loss of the IBBCEAS system are evaluated. Using the difference of Rayleigh scattering between nitrogen and helium, the reflectivity curve of the high-reflection mirror is obtained. The reflectivity R of the mirror at 436.1 nm of the IO absorption peak is about 0.99982, and the effective absorption optical path reaches 3.83 km under vacuum condition. According to the absorption of O₄, the effective cavity length of the modified system is 60.7 cm. The Allan deviation is used to evaluate the performance of the system, and the standard deviation is used to analyze the detection sensitivity of the system. When the time resolution is 60 s, the detection sensitivity (2σ) of the system for IO and NO₂ are 1.9 pptv and 20 pptv (part per trillion by volume), respectively. The iodine dissolved in potassium iodide (KI) solution is taken out by the bubbling method and react with ozone after photolysis to produce a stable concentration of IO sample gas. The IO loss in the sampling tube is calibrated, and the results show that the sampling tube has no significant effect on the IO loss. The IBBCEAS system is used to determine the linearity of IO, and the correlation coefficient R² between the measured concentration of IO and the proportioned concentration in a concentration range from 39 to 530 pptv is 0.99. The IO produced by the reaction of iodine released from kelp with ozone is measured.