Cryogenic target is one of the key components of inertial confinement fusion. The removal degree and efficiency of impurity gas in cryogenic target are of great significance to the on-line preparation of ice layer for cryogenic target fuel. According to the design requirements of cryogenic target physics for impurity content in ice layer, the influence factors of upper limit of partial pressure are analyzed, based on the derivation of the calculation formula of maximum allowable partial pressure of impurity gas in the target. Then the flow field of air and hydrogen in microchannels is investigated, and the filling and evacuation model of gas flow in a microscaled filling tube is established. The dynamic simulations of microtubules with different lengths and diameters are carried out. The results show that the microtubules with a length of 5 mm could save 80% of the time compared with the microtubules of 50 mm in length when the microtubule is 5 μm in diameter. At the same time, the total flow washing time decreases by 46% when the diameter of 2 μm is doubled under the condition of 20-mm-long microtubule. Considering the requirements for efficiency and fusion stability, four kinds of tubes are proposed and simulated. The results indicate that the conical transition tube has a strong flow capacity and high flow evacuation efficiency, and is suitable for use as a filling microtubule. On the basis of the best tube shape, the comparison between the two processes under different intermediate pressures is carried out with the time and number of filling and evacuating serving as evaluation criterion. Ultimately, the intermediate pressure of 52000 Pa is selected, the total number of evacuation is 10 and the time is 758 s. Finally, the effect of temperature on the evacuation efficiency is studied in a temperature range of 113 K－293 K in steps of 60 K. The results show that the total time of filling and evacuation will be reduced by 15% on the basis of normal temperature when the temperature is reduced by 60 K, which proves the feasibility of evacuation at low temperature in practical operation.