Abstract: We described a new and simple on-line pyrolysis method for the analysis of δ¹⁸O and δD in oil-and-gas field water by continuous flow EA-IRMS.The sample water was converted into CO and H₂ in glassy carbon tube which was filled with glassy carbon granules at high temperature (1 450℃).And then,CO and H₂ were separated in GC column prior to transferring into the ion source of the Isotope Ratio MS for sequential determination of hydrogen and oxygen isotopes of the sample by magnet switching.This method performed dual measurements of δ¹⁸O and δD from a single sample,and had high efficiency and good productivity.Using this method,we measured some water samples from gas fields in the Sichuan Basin.The results showed that the water of Triassic Xujiahe Formation was the mixture of marine sedimentary and ancient meteoric water.