In order to quickly screen for the presence of ¹⁴C in target water body and provide quantitative measurement results, a rapid and accurate analysis of ¹⁴C activity concentration in samples was achieved by optimizing the counting region of the liquid scintillation spectrum and improving counting efficiency with an internal calibration method under quenching effects, resulting in a lower detection limit of 1.8 Bq/L. This method was then applied to determine the ¹⁴C activity concentration in the samples from the deday pool of a nuclear medicine factory and the surrounding environmental water. The results showed that the activity concentrations of the three samples were below the lower detection limit, whereas those of the others were within 38-1 332 Bq/L. The accuracy of this direct analysis method was verified by using self-prepared samples with known activity concentrations, which showed a maximum deviation of 1.1% from the reference values.