A gas-filled time-of-flight (GF-TOF) detector system for isobaric identification has been developed at the China Institute of Atomic Energy (CIAE). The time resolution of the old GF-TOF detector built in 1999 by Jiang et al was about 1.3 ns without gas filled, which is not so good as expected. Later, an improved detector was built and tested by ions of ³⁶Cl with the energy of 28 MeV in 2002, but the results were still not satisfactory. Recently, some further improvements have been achieved. The newly built GF-TOF detector was tested by using a ³⁶Cl standard sample (³⁶Cl/Cl=7.6X10^-9 g/g) with the ³⁶Cl ion energies of 64, 49 and 33 MeV, respectively.Time resolutions of 350 ps, 580 ps and 920 ps were obtained for 64, 49 and 33 MeV ³⁶S, respectively, without gas filled. ³⁶Cl and ³⁶S particles were successfully separated in the TOF spectra from the GF-TOF detector at the three different incidence energies mentioned above. The dependence of time resolution and separation power of GF-TOF method on the incidence energy and the residual energy is discussed. The comparison of separation power for isobars between the GF-TOF method and the ΔE-E method is described. A combination of GF-TOF method and ΔE-E method may further improve the separation power for isobars, especially at low energy. The results show that the sensitivity for ³⁶Cl AMS measurements is 10^-13 g/g at the energy of 33 MeV. Some results relative to the GF-TOF method are given.