Abstract: The value of activity concentration of tritium standard source is very important for the monitor and calibration of tritium. The length compensation method is applied for measuring activity of tritium. The analysis and uncertainty evaluation results of measuring uncertainty is given based on internal inflatable proportional counter The uncertainty of the activity concentration of tritium mainly came from the statistical fluctuation of background and count, volume measurement, dead time correction, wall effect correction, and measurement repeatability and stability. The results show that the standard uncertainty of synthesis is 1.6% for 7×10³ Bq/L tritium gas.