Abstract: The non-uniformity of the gas molecule density distribution in the continuous gas expansion vacuum standards is the cause of the error of the metrical value of the gauge to be calibratedby.It’s the also the source of systematic error of these standards.This paper presents theoretical calculation method of molecule flow distribution in calibration chamber.This method not only can entirely analyse the non-uniformity of the gas molecule density distribution,but also can calculate the molecule density in the calibrated gauge volume.It indicate the function relation between the molecule density in the calibrated gauge volume and the throughput enter into the calibration chamber.Thus minimizing the drawbacks of the geometric structure influence of the molecule density distribution.The calculation method of the backstreaming coefficient β and effective pumping speed S_eff present theory foundation for the calibration system parameter Monte Carlo simulation.