Based on the flow theory applicable to the whole Knudsen number range, a theoretical model for removing air from the target shot in inertial confinement fusion was established, and the reliability of the model was verified by designed experiments. The physical experiment requires the air concentration in the target capsule to be lower than 10×10^-6, the process of removing air in the capsule was simulated numerically, and the relationship between the air concentration in the capsule, the pressure in the capsule and time was emphatically analyzed. The time consumed by three methods for removing the air in the capsule, namely the single-pipe one-time gas evacuation method, the single-pipe circulation gas evacuation method and the double-pipe flow washing method, was calculated and compared. Numerical calculation results show that: in the single-pipe one-time gas evacuation method, the existence of the micro-channel on the capsule has a non-negligible effect on the time required to remove the air in the capsule, and it takes 1961.77 h for the air concentration in the target shot to reach the standard when the micro-channel on the capsule and the gas-filling pipe is considered. In the single-pipe cycle gas evacuation method, the number of evacuation times and the degree of single gas evacuation will affect the total time required to remove the air in the capsule. When the single gas evacuation degree is at the optimal value, the scheme that filling three times and evacuating four times can reduce the total time to reach the standard to about 1 h, while the single gas filling time and gas evacuation time are 6 min and 10 min, respectively. However, it takes only 11 minutes for the air concentration in the capsule to reach the standard by using the double-pipe flow washing method.