Abstract: The theoretical and experimental study on the purge tube length effect in the air purge method was carried out. Two problems including the effect of the purge tube length on the pressure amplitude and on the pressure phase delay were mainly investigated. The results show that the tube length has little effect on the pressure amplitude when it is no bigger than 17 m. Obvious attenuation is found out when the tube length is bigger than 17 m and the attenuation coefficient calculated by the theoretical model shall be used to calculate the real pressure amplitude. The tube length has obvious delay effect on the pressure wave. The delay time increases with the tube length. The pressure wave delay has big effect in a pressure difference measurement when the lengths of the two purge tubes are not equal. The gas velocity in the purge tube calculated by the theoretical model provides a good method for the evaluation of the purge cup volume. A purge cup with volume of 50 mL could satisfy the requirement of the air purge method. The theoretical and experimental results in this article provide fundamental for the application of the air purge method.