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Ra m seawater was measured by Mn-tiber adsorption spectrum method. Because that the sample could not be measured immediately after sampling, the activity decay correction must be done. The formula of calculating 224Ra activity at sampling time from spectrum data was deduced, which included the correction of unsupported 224Ra decay and build up 224Ra from 228Ra and 228Th decay. 224Ra was measured by y-rays of its daughter 212Pb and 2O8T1 mean the sample must be sealed for prevent medial nuclide 220Rn escape and assure the equilibrium relationship of 224Ra and its daughters, so the important reference times are sampling, sealing and spectrum acquisition times. The formula gives the activity at sealing time and the activity at sampling time is calculated by simple exponential decay correction. As application, we measured 224Ra in the seawaters collected from the western South Yellow Sea. At the sealing time, the 224Ra activities in samples are 0.27-13.8Bq, but 224Ra activities from 228Ra and 228Th decay are 5.8-88.6％ of total 224Ra, that means