To address the technical challenge of maintaining a constant bandwidth during the center frequency tuning process in traditional tunable filters, a new balanced bandpass filter with adjustable center frequency, constant bandwidth and high frequency selection performance is designed. Based on the electromagnetic coupling between the resonators, three coupling structures are formed through four tunable oscillators, and the coupling between the source and the load on the feed circuit is introduced, so that two transmission zeros are generated on each side of the filter passband. Using only the same DC bias voltage to load on the transformer diode, the center frequency of the filter is adjustable. Utilizing odd-even mode analysis theory and the target curve of coupling coefficient and external quality factor varying with the resonance frequency, the design objectives of constant bandwidth, low insertion loss and good out-of-band suppression are achieved. The simulated and measured features of the filter are in good agreement, with an adjustable central frequency range from 788 to 978 MHz, while sustaining a 3 dB bandwidth of (43±1.2 MHz) throughout this range. Compared to existing technologies, this design effectively reduces device size through optimized folding of resonator structures while maintaining constant bandwidth, providing a feasible solution for the realization of high-performance tunable filters.