Abstract: Constant conductance method uses quardrupole mass spectrometer to compare the known gas out of conduction orifice with gas from vacuum leak,It is a very precise method to deal with leak rate.Orifice is the use of machinery,laser processing method for producing,In the molecular flow conditions,the orifice conductance is a constant.Use this method for calibration data,we found a standard vacuum leak detection.To ensure the credibility of calibration data,the uncertainties of calibration system should be precisely estimated.