Abstract: The water vapor can affect the measuring result of the hydrogen isotopic composition which was attained by improved measuring method for trace hydrogen.It was proved by experiments that standard deviation was 17.15 for 6 measured results of trace hydrogen,whose content was 0.1% before the chromatographic column was heated to remove the water vapor,it was suggested that although the measured result had an average which was similar to the generally accepted value for the employed hydrogen in the experiment,the measured result had a wide distribution range and each single measured result had a relatively weak reliability.After the chromatographic column was heated to remove the water vapor,the 6 measured results for the same sample had a standard deviation of 4.56,which suggested that the measured results were relatively concentric.It was safely to conclude that heating the chromatographic column can significantly improve the reliability of the measured result.What's more,the average value could not be used as the real isotopic composition for lower content hydrogen gas,the measured result should be assessed taking the standard deviation value into account.