A single-stage ring resonator capable of introducing six modes within the ultra-wideband (UWB) passband is pre-sented.The sextuple-mode resonator consists of three rings and three sets of stepped-impedance open stubs.Based on this sextuple-mode resonator,a UWB filter fed by the interdigital-coupling line (ICL) is designed.And we propose a two-round interpolation method to obtain the filter's initial dimensions.The designed filter is fabricated on a double-sided YBCO/MgO/YBCO high-temperature superconducting (HTS) thin film for demonstration.The experimental results show that this UWB filter produces eight resonances in the passband eventually,which effectively improves the in-band reflec-tion and the band-edge steepness.Moreover,the upper stopband performance is enhanced due to the transmission zeros(TZs) generated by the stepped-impedance open stubs and the ICL structure.The measured good performance verifies the practicability of the two-round interpolation approach,which can also be extended to other odd-even-mode filter designs.