A filter includes a plurality of resonance cavities, where a resonance tube and a tuning bolt penetrating into space enclosed by the resonance tube are disposed in each resonance cavity; further includes a tuning part disposed between the tuning bolt and the resonance tube, where the tuning part and the resonance tube form a first capacitor, and the tuning part and the tuning bolt form a second capacitor; and further includes an adjusting structure used for rotating the tuning part, so as to change relative areas between the tuning part and the resonance tube and between the tuning part and the tuning bolt, so that the first capacitor and the second capacitor change synchronously. In the present invention, the tuning part is disposed between the tuning bolt and the resonance tube, so that the tuning part forms a double-capacitor structure with the resonance tube and the tuning bolt.