The invention discloses a filter used for a ventilation system. The filter comprises a filter screen, wherein a ceramic composite material is coated on the filter screen; the ceramic composite material consists of the following ingredients in percentage by weight: 45 to 54 percent of polar mineral tourmaline with the particle size of 0.1 to 1.5 microns, 3 to 9 percent of rare earth nitrate or rare earth oxide and 43 to 52 percent of transition metal oxide. In the filter, the filter screen is coated by adopting the ceramic composition material which can generate negative ions so as to ensure that the air enters a house through the filter screen keeps enough oxygen, enhance air filtering capacity and protect the ventilation system at the same time. A filter material is a metal composition screen material, which has the characteristics of high strength, wind erosion resistance, rust resistance, washability and the like.