The bag fine filter is mainly used to filter and remove ≥1μm dust particles in the air conditioning system, protect the subsequent sub-high efficiency or high-efficiency filter, and prolong their service life. Based on different filter performance requirements, the filter media of bag filter may be multi-layer compound synthetic fiber bucket materials, synthetic fiber non-woven fabrics of different thicknesses, glass fiber cotton, etc. The efficiency stays constant even with varying airflows. Also, the bags are intrinsically stiff while being exposed to the airstream. In order to guarantee proper stability even with over-size filters.