Embodiments of the present invention disclose a filter, including: a conductive box body, and an insulating substrate, a first conductor, and a second conductor that are arranged inside the conductive box body. The insulating substrate includes a first surface and a second surface. The first conductor is arranged on the first surface of the insulating substrate. A position on the second surface corresponding to the first conductor contacts with the conductive box body. The second conductor is arranged on the first surface or the second surface of the insulating substrate. The second conductor and the conductive box body form a coaxial resonant cavity together. Further, an end of the second conductor is coupled with the first conductor, and the other end of the second conductor is coupled with the conductive box body. The filter disclosed in the embodiments of the present invention has advantages of a microstrip filter of simple manufacturing process and small volume, and further has advantages of a coaxial cavity filter of high Q (power factor) value, small insertion loss, and large power capacity.