Abstract: A clear picture of the changes in combustion transformation capability (CTC) of EA-Conflo-IRMS with the number of samples was measured and the relationship of measured nitrogen and carbon isotope ratios with their CTC were depicted by analysis of the data accumulated in the long time of work. The CTC displays little variation during a period of first 356 runs, followed by the degression of CTC for the subsequent 127 samples and the ensuing relatively lower level of CTC for next 65 runs. This marked decrease in CTC occurred when most Cu in reduction tube was oxidized and the oxidative ability of upper part of combustion tube declined. Moreover, the measured nitrogen and carbon isotope ratios tend to reduce with the declined CTC despite there are some complicated changes in the measured isotope ratios due to the influences of other factors. In order to normalize the shifted state of this machine, a reversed calibration was performed on the CO₂ reference gases by some standards, determined when EA-Conflo-IRMS at its lowered CTC level, to give out the properly normalized δ values, relative to which the carbon isotope ratios of some standards were measured again. The accuracy (less than 0.1‰) and precision (better than 0.15‰) indicate the practicability of the above method.