Abstract: In this paper,a novel coupling structure used in compact coaxial-cavity tunable filter was proposed and discussed.The absolute bandwidth of tunable filter with this coupling structure can remain almost constant within the entire operating frequency band.This coupling structure was composed of a rectangular window and a triangular loop.It could restrict the variation of the coupling bandwidth（CBW） versus the operating frequency in the base of the coupling coefficient between cavities was ensured.As the result,the absolute bandwidth of the tunable filter could be kept almost constant within the operating frequency range.According to the study of EM field distribution in the quarter-wavelength coaxial-cavity,the constraint of structural parameter was acquired in which the tunable filter can remain almost constant bandwidth within the operating range.A 3-cavity tunable coaxial-cavity filter adopting the proposed coupling structure was designed and fabricated,and the tuning range of 56% and bandwidth variations of ±3% were achieved.