Abstract: In this paper, a new structure is proposed for switchable constant absolute bandwidth frequency tunable bandpass filter, keeping the characteristics of absolute constant bandwidths before and after bandwidth switching. Based on coupling coefficient extraction method and filter synthesis theories,we designed and fabricated an open loop resonant microstrip tunable bandpass filer operating in 1. 3 to 1. 45 GHz with switchable bandwidth. The measurement results show the 3 dB passband bandwidth switches as 120MHz, 100MHz and 80MHz respectively in the operation frequency range, meanwhile the insertion loss is -2. 9dB, -2. 4dB, -2. 6dB correspondingly. Simulation results fit well with measurement results. The proposed filter has the advantages of simple structure, low fabrication difficulty, minimized size and convenient feed controls.