The compressed air from the air source contains a large amount of liquid water, solid dust with different particle sizes, oil dirt, oil vapor, etc. If these impurities are allowed to enter the freeze dryer directly, the working condition of the freeze dryer will deteriorate. For example, oil pollution will pollute the heat exchange copper tubes in the precooler and evaporator, affecting the heat exchange; Liquid water increases the working load of the freeze dryer, and solid impurities are easy to block the drain hole. Therefore, it is generally required to install a pre-filter upstream of the air inlet of the freeze dryer for impurity filtration and oil-water separation to avoid the occurrence of the above situation. The filtering accuracy of the prefilter for solid impurities is not very high, generally 10-25 μ OK, but it is better to have a higher separation efficiency for liquid water and oil pollution. Whether the post-filter of the freeze dryer is installed shall be determined by the user's quality requirements for compressed air. For general power gas, a main pipeline filter with high accuracy is sufficient. When the requirements for gas consumption are higher, corresponding oil mist filter or activated carbon filter shall be configured.