A filter, a communications apparatus and a communications system are disclosed. The filter includes: a top panel, a bottom panel, and two first side panels located between the top panel and the bottom panel, where the two first side panels, and parts that are of the top panel and the bottom panel and are located between the two first side panels form a first rectangular waveguide; and at least one diaphragm that has a metal surface, where the at least one diaphragm that has a metal surface is connected to both the top panel and the bottom panel and separates a cavity of the first rectangular waveguide into several cavity chambers that extend along an input/output direction of the first rectangular waveguide; where at least one of the first side panels is an adjustable side panel whose position relative to the diaphragm is adjustable. Adjustment of an operating band of a filter may be achieved by merely adjusting a position of an adjustable side panel, without a need of replacing a related device, and therefor
