Abstract: Constant conductance method uses partial pressure measurement technology,which has no strict request to outgassing rate of mass spectrum analysis chamber.By experimental research,the measurement error caused by non-linearity of quadrupole mass spectrometer is up to 38%.To eliminate error,the gas pressure in ballast chamber can be adjusted precisely to make gas flow through the orifice very near or equal to the leak rate of calibrated vacuum leak.Moreover,the gas pressure in ballast chamber is high enough that pure single gas can be realized by no strict material disposal and no bakeout.The evaluated uncert ainty is 2.6%for calibration of vacuum leak by constant conductance method,which can be further decreased through precisely calibrating capacitance diaphragm gauge and controlling temperature.