Abstract: The principle and method ofmeasuring conductance with PID constant pressure control mode and sawtooth variation constant pressure control mode are introduced. The comparison of two measurement method are done. The two agree fairly well, the difference is less than 0.14%。Through the experiment of leak calibration, it has been proved that experiment result is more accurate than theory calculation. For the irregular small orifice in shape, the experiment method of measuring conductance has the practical value.