An analytical method was developed for the determination of ultralow level 135Cs in environmental samples by chromatographic separation of cesium with AMP-PAN and AG50W-X8 columns and sensitive measurement of cesium isotopes with triple quadrupole inductively coupled plasma mass spectrometry (ICP-MS/MS). Cesium was simply released by acid leaching using aqua regia from environmental solid samples and preconcentrated on AMP-PAN column. The cesium adsorbed on the column was effectively eluted with NH4Cl solution without dissolving the AMP. The excessive amount of NH4Cl in the eluate was removed by sublimation in the presence of small amount of LiCl. The remaining barium and other interfering elements such as Mo, Sn, Sb, and Li were efficiently removed using cation exchange chromatography (AG50W-X8). The decontamination factors of this procedure are above 4 × 107 for barium and 4 × 105 for molybdenum; the chemical yields of cesium are more than 85% for samples of less than 10 g. This method enables to separate cesium from large size of samples for the determination of ultralow level 135Cs, avoiding the problem of removal of a huge amount of Mo in the dissolved AMP. Intrinsic 137Cs in the environmental samples measured by gamma spectrometry before and after separation was used as internal isotope dilution standard for quantitative determination of 135Cs without complete release and recover of radiocesium. The interference of barium (135Ba and 137Ba) to the ICP-MS measurement of 135Cs and 137Cs was further suppressed to 8 × 10-5 by using N2O as the reaction gas in ICP-MS/MS at a flow rate of 0.7 mL/min, so a total suppression of 2 × 10-12 for Ba was achieved, making the isobaric interference of Ba isotopes to the measurement of 135Cs and 137Cs in environmental samples negligible. A detection limit of 9.1 × 10-17 g/g for 135Cs and 137Cs was achieved for 60 g samples. The developed method was validated by analysis of standard reference materials (IAEA-375, IAEA-330, and IAEA-385) and successfully applied for the determination of 135Cs concentrations and 135Cs/137Cs ratios in soil samples collected from Denmark, Sweden, and Ukraine. The 135Cs/137Cs isotopic ratios in Danish soil (2.08-2.68) were significantly higher than that from Sweden and Ukraine (0.65-0.71), indicating different sources of radiocesium. This work demonstrated the application of 135Cs/137Cs as a unique fingerprint for discriminating the sources of radioactive contamination and estimating their contribution to the total inventory, which will be useful for nuclear forensics and environmental tracer studies.