Abstract: Hidex 300SL is the liquid scintillation device based on triple-to-double coincidence ratio(TDCR) method for absolutely measuring the activity of low level tritium water. The relationship between the detection efficiency of the system and the TDCR was established by an unquenched standard tritium water sample measurements. The influences of counting rate, disintegrations rate and TDCR were also discussed. Additionally, a detailed optimization study of the instrument operation parameters was carried out. The system detection efficiency of 73.2% and the stability better than 0.47% were obtained with “H-3” measurement model and 35 ns coincidence gate. Finally, the detection limits were evaluated by an air background sample measurements, which can provide useful references for the accurate determination of tritium concentration in environmental water.