Embodiments of this application disclose a filter, a phase shifter, and a related apparatus. The filter includes: a cavity, a circuit board, and a filter branch. The circuit board is located in the cavity. A power divider circuit is disposed on the circuit board, and the power divider circuit includes an input end, a main feeder, and an output end, where the input end is electrically connected to the main feeder, the output end is electrically connected to the main feeder, and the main feeder is configured to transmit a signal that is input at the input end to the output end. The filter branch includes a first interface and a first filter line, where the first interface is disposed at an opening of the cavity, the first filter line is located outside the cavity, the first filter line is electrically connected to the first interface, and the first interface is electrically connected to the main feeder. The first filter line in the filter branch is disposed outside the cavity. Therefore, a user may adjust a length of the first filter line outside the cavity, to adjust a wavelength range of an electromagnetic wave filtered out by the filter. This improves user convenience.