Electrochemical methods are the main methods for measuring seawater pH. The self-developed electrochemical pH sensor is based on iridium metal and its oxide electrode, a method for its calibration and data correction in seawater environment was established and applied for in-situ testing in nearshore and oceanic environments. The instrument calibration includes (1)Selecting appropriate calibration buffer reagents according to the calibration requirements of seawater pH analysis; (2)Preparing calibration buffer system by replacing the commonly used 2-aminopyridine (AMP) solution with standard seawater. Data correction mainly includes temperature background correction and error correction. Insitu testing in the sea area and comparison with other similar instruments show that the error caused by the difference of calibration system can be up to 1.00 pH unit, and the data correction can improve the testing accuracy from 0.10 pH unit to 3.00 pH unit. The calibration and data correction method of the instrument can effectively improve the measurement accuracy of this self-developed pH sensor.