An improved version of the bag filter, which filters paint mist in the air based on the principle of filtration. The filter medium is composed of two layers of filter cotton. The first layer of filter cotton has diamond-shaped holes, and the windward side forms a 3D structure of the windward side. During the process of air carrying paint mist passing through the filter medium, the paint mist is filtered and blocked on the windward side. The structure of the wister-shaped holes on the windward side has a filtration area twice that of the flat filter cotton, effectively increasing the paint holding capacity of the filter box. The final filter cotton can also ensure a high filtration efficiency.