Abstract: A simple technique using three step cation-exchanges （Ba-type, H-type, and Cs-type）was used to enrich and purify CI from the sediment porewater samples. The recovery of CI is better than 99.6%. After purification, the ^37CI/^35CI ratios of porewater samples were measured using positive ion thermal ionization mass spectrometry based on the measurement of Cs2Cl^＋ ion with graphite load. The measured results for both the NaCI standard and one porewater sample show that the method has high stability and high precision （0.09 -0.13‰）. Initial results for porewater sampled from the South China Sea reveal significant -δ^37 Cl variation, and it may have great implications for further gas hydrate exploration in the area