Abstract: During the operation of the Tokamak device, impurity particles adsorbed on the first wall are released into the inner vacuum chamber, affecting the smooth operation of the plasma. Therefore, it is necessary to perform wall treatment on the device before discharge. A differential extraction measurement system is designed that can reduce gas pressure while maintaining the same proportion of gas in the inner vacuum chamber, achieve the normal operation of a quadrupole mass spectrometer, and monitor gas composition during wall treatment. The entire design process is carried out around the conductance design of the key component a small conductance flange, and the gas balance method is used to calculate the design parameters of the small conductance flange. It is possible to reduce the pressure of the test chamber from 10 Pa to about 0.01 Pa in the molecular flow state. The results provide reference for the design of gas composition monitoring equipment under similar high-pressure conditions.