The iodine-to-calcium (I/Ca) ratio in marine carbonates can indicate changes in seawater redox. The use of inductively coupled plasma mass spectrometry (ICP–MS) to determine the I/Ca ratio in carbonates is challenging owing to low iodine content, volatility, and significant Ca matrix effects. In this study, an online extraction device was designed to evaluate the influence of extraction time on the detection of iodine signals. The intensity of I signal increased from 0 to 3 min after the addition of 2% HNO3, and a stable I signal was obtained from 3 to 8 min. After 8 min, the analytical signal decreased owing to the volatilization of iodine. Experimental results revealed a linear relationship between the intensity of I/Ca ratio and the concentration of I/Ca ratio, indicating that Ca can serve as an excellent internal standard (I/Ca ratio) to correct the attenuation of the I signal caused by Ca matrix effects. Therefore, a novel online acid extraction method coupled with ICP–MS was developed by strictly controlling extraction time (3–5 min) and using the I/Ca ratio for direct correction to obtain the final I/Ca ratio, with a detection limit of 0.02 μmol/mol and relative standard deviation ranging from 1.5% to 10.3%. The proposed method was applied to determine the I/Ca ratio in carbonate reference materials. The measured I/Ca ratios were consistent with the reference values.