The design of a notch reconfigurable ultra-wideband filter based on a multimode resonator is presen ted in this paper. The structure of the multi-mode resonator is designed based on transmission line theory and odd even mode theory. The dimension parameters of the filter are determined via the joint simulation of HFSS and ADS. The notch of the filter is reconfigurable by loading varactor-diodes on the filter branches. The center frequency of the designed ultra wideband filter is 3.35 GHz, with a 3 dB relative bandwidth of 68.7%, and the notch frequency is independently adjustable. The simulation results show that the adjustable range of notch frequency in the pass band is 3.2-4 GHz, with a relative adjustable range of 33%. The insertion loss in the passband is within 0.5 dB. After processing and actual measurement of the filter, the tunable range of the notch obtained is 3.3-4.02 GHz, which is consistent with the simulation results. The designed reconfigurable ultra-wideband filter has a simple struc ture, small size, wide notch adjustable range, and can effectively suppress interference signals in the system.