A narrowband cavity filter with high suppression and miniature structure is designed and realized. By using the principle of loading capacitance, a rectangular metal column is added to one side of the cover plate, which increases the coupling capacitance and reduces the distance between adjacent resonant cavities, thereby realizing miniaturization of the filter. By CST simulation, a narrowband cavity filter with a center frequency of 7.325 GHz and a bandwidth of 0.25 GHz is designed. The final physical test shows that the standing wave ratio in the band is good, the outofband rejection is high, and the physical size is also significantly reduced.