Due to increasing application of 210Po/210Pb in studying particle dynamics, a consistent procedure and calculation to derive accurate and precise result of 210Po and 210Pb in seawater should be proposed in the framework of intercalibration by GEOTRACES. The associated uncertainty of radioactivity, which is a significant component of data report, plays a vital role in intercomparison and should be well evaluated. Although measurement uncertainty of laboratory result was well defined in ISO standards and IAEA technical documents, the decay/ingrowth uncertainty correction from laboratory result to in-situ result was less studied. It was demonstrated that the relative uncertainty of in-situ 210Pb activity was independent of elapsed time and equal to relative uncertainty of laboratory measuring 210Po activity at second spontaneous deposition date. The relative uncertainty of in-situ 210Po activity decreases with in-situ activity ratio of 210Po to 210Pb and increases with elapsed time between sampling date and separation date, relative uncertainty of laboratory measuring 210Po activity at first spontaneous deposition date and relative uncertainty of in-situ 210Pb activity. It was more important to improve precision of 210Po at first spontaneous deposition date than that of 210Po at second spontaneous deposition date. To obtain a desirable relative uncertainty of in-situ 210Po activity, the maximum allowing elapsed time for 210Po, which was important for sampling strategy making and quality assurance, was calculated by in-situ activity ratio of 210Po to 210Pb and precision of analytical method for 210Po. The methodology of decay/ingrowth uncertainty correction could also be applied for other radionuclide pairs (234Th/238U, 90Y/90Sr, 210Bi/210Pb), sample matrixes (aerosols), and disciplines.