Abstract: In order to measure tritium activity in sea water with lowlevel liquid scintillation counter PE Quantulas 1220 spectrometer， we studied the measurement time， appropriate background of water， scintillation cocktail and sample/scintillation ratio. The minimal detectable activity of this method is 1.07Bq/dm3 after optimizations. In order to test the reliability of the method， we participated in the comparison measurement of IAEA seawater samples. The result of our sample is 2.91±035 Bq/dm3 which is only 3.9% to the relative deviation of IAEA reference value （2.80±0.06 Bq/dm3）. It proves the feasibility of our method. Compare with the value reported， it is considered that the method is suitable for tritium measurement in waters adjacent to the nuclear power station．