Some improvements of the detection sensitivity in pulse time interval analysis (TIA) based on selective extraction of successively α-α correlated decay events within millisecond order from random or background events, were established by the utilization of PSD, to reject β/γ-pulses from α-ones and a simple chemical procedure of radium separation, together with the use of well resolved scintillator. By applying the PSD, the contribution of β-decay events was completely eliminated in both the α-spectra and the TIA distribution curves as well as the improvement into clear energy resolution and the enhancement of detection sensitivity for the TIA. As a result, the TIA and α-spectrometric analysis of 2 2 6 Ra-extract showed the existence of 2 2 3 Ra (Ac-series) and β/α-correlated events with correlated life (due to 0.16 ms due to 2 1 4 Bi(β)→ 2 1 4 Po(α)→) along with a singly well resolved α-peak to be useful for the determination of 2 2 6 Ra (U-series). The difference of half-lives (145 and 1.78 ms) due to 2 1 6 Po and 2 1 5 Po (direct daughters of 2 2 4 Ra for Th-series and 2 2 3 Ra for Ac-series, respectively) was also proven for the possibility of the simultaneous determination of both correlated events by using the TIA/PSD combined with chemical separation and liquid scintillation counting method. Finally, the simultaneous determination of three natural decay series, which include U-, Th- and Ac-series nuclides, have been conveniently carried out for some environmental samples using the present method combined with 2 2 5 Ra yield tracer (Np-series).