Seawater calcium ion (Ca 2+ ) concentration was investigated based on the potentiometric titration method during the summer of 2018 in the Bohai Sea, China. The measured Ca 2+ concentration ranged from 7760 to 9739 μmol kg -1 and deviated from the theoretical Ca 2+ values, which were estimated from the calcium/salinity ratio. The excess calcium (Ca 2+ excess ) ranged from 186 to 1229 μmol kg -1 , showing a decreasing trend from the estuary to the nearshore, and then the offshore areas. Riverine input was an important source of seawater Ca 2+ excess in the Bohai Sea. Biological activity was another factor in regulating seawater Ca 2+ excess by precipitation in the Yellow River estuary and dissolution in other area of the Bohai Sea. Furthermore, the aragonite saturation state (Ω arag ) values calculated from the measured Ca 2+ concentrations showed a significant deviation from the values calculated from the theoretical Ca 2+ concentrations, especially in the estuarine area with a maximum difference of 18.5%. Therefore, the disregard of the calcium addition would lead to an underestimation of the calcium carbonate saturation state and a deviation in the assessment of ocean acidification in marginal seas.