Abstract: The effect of sampling pipline and physical process on partial pressure mass spectrum analysis was studied.The diffusion equation was established and the function of analyzed gas pressure in mass spectrometer chamber with time was gained.Analyzed gas pressure is the complex exponent Increasing with time,and becoming constant after long time. As resulte of long sampling pipeline,the ratio between the analyzed gas pressure and air pressure became N times which was related to gas mass. N was also related to time and length of sampling pipeline in short time, but after long time, the effect of time and sampling pipeline length can be neglected. The gas constant time t_c=max（3l²/R₀vπ²,2τ）