The previous work on reflective linear polarization converters has mostly focused on wideband characteristics, while the out-of-band selectivity has received rare attention. To achieve wideband and high selectivity simultaneously, a wideband reflective linear polarization converter with quasi-elliptic filtering characteristic is proposed based on multi-mode resonant metamaterial. Through the loading of the cross-shaped slotlines and inter-unit connecting lines, the inherent degenerate modes of a square patch can be excited and separated, and two additional resonant modes and two independently controllable conversion zeros are introduced to achieve excellent wideband and quasi-elliptic filtering characteristics. After the principles of generating in-band resonant modes and out-of-band conversion zeros are revealed, the mechanism of polarization conversion is analyzed in detail from the perspective of the amplitude-phase imbalance between its polarization components. Finally, a polarization converter is designed, fabricated, and measured. The main-polarization reflection coefficient is less than − 10 dB in fractional bandwidth of 50%, wherein the polarization conversion rate is 90%, and the polarization conversion rate squareness is about 0.9. The proposed converter with excellent frequency selectivity provides great potential for applications in multi-band and multi-functionality systems for reduced isolation band.