A resonator, a filter, a duplexer, a multiplexer, and a communications device that use the resonator, where the resonator includes a resonant cavity body that has a resonant cavity and an open end, a cover that covers the open end and that is connected to the resonant cavity body, and a resonant tube that is located inside the resonant cavity, a medium material is padded in a capacitor area in the resonant cavity and whose dielectric constant is greater than 1, the resonant tube includes a resonant tube body and an elastic structure that is combined with the resonant tube body, and the elastic structure provides elastic pressure in an axial direction of the resonant tube where the resonator may reduce a conductor loss and improve a power capacity, and has relatively low costs.