Abstract: Seawater Ca²⁺concentration is one of the important parameters for calculating calcium carbonate saturation, which is usually derived from seawater calcium-salinity ratio, but this may not be applicable in offshore waters because of influences of many factors. In this study, the accuracy and precision of Ca²⁺concentration determined by automatic potentiometric titration at different salinities were investigated and the potential influence of the “salt effect” was evaluated. The difference between the measured Ca²⁺ concentration and the estimated value by seawater calcium-salinity ratio in the offshore mariculture area was compared. The results showed that: (1) the precision of Ca²⁺concentration determined by automatic potentiometric titration was high at different salinities, with the standard deviations of five replicate determinations ranging from 0.001 to 0.006 mmol/kg, which were better than 0.1 %. (2) In the salinity range of 20.00-34.62, the relative errors between the measured and calculated values based on the calcium-salinity ratio of Ca²⁺ were -0.043 %-0.023 %, which were within ± 0.05 %. (3) The measured and calculated values of Ca²⁺concentration in seawater samples at different salinities agreed with each other, indicating that there was no “salt effect”. (4) The measured values of surface and bottom seawater Ca²⁺ concentration in Muping Marine Ranching were higher than the calculated ones i.e. 0.360 mmol/kg and 0.333 mmol/kg, respectively, which might result from the impact of riverine input. Therefore, calculating the calcium carbonate saturation of offshore seawater by measured Ca²⁺ concentration can more accurately assess the extent of offshore acidification.