The design of a notch reconfigurable ultra-wideband filter based on a multimode resonator is presented 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 passband 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 structure, small size, wide notch adjustable range, and can effectively suppress interference signals in the system.