There are several experiments for memory effect analysis and post-processing method analysis carried on in this study, using a liquid water isotope analyzer based on the theory of off -axis integrated cavity output spectroscopy (OA -ICOS). The performance characteristics and impacts on the determination results from the analyzer of memory effect were analyzed through these experiments. And the post -processing methods in the hydrogen and oxygen isotope determination were discussed. The results indicate that the memory effect is significant in hydrogen isotope determination. However, the performance of the memory effect is not obvious in oxygen isotope determination using conventional method. Basically, the accuracy of liquid-water isotope analyzer can be ensured in hydrogen and oxygen isotope determination using the conventional methods. It is shown that the accuracy of hydrogen and oxygen isotope analysis can be improved by increasing determination times. It is an effective way to improve the efficiency and accuracy of isotope determination and save the experimental costs by choosing three standard samples which δ values are most closely to that of the samples in hydrogen and oxygen isotope determination and post-processing.