In this article, a wideband high-selectivity filtering linear polarization converter is proposed and applied for ultranarrow isolation-band bifunctional metasurface. As the contribution of this work, the developed polarization converter based on slot-line loaded triangular resonator unit not only achieves the in-band multimode resonant characteristic, but also introduces two additional controllable zeros out-of-band, so as to enable simultaneous implementation of the wideband and high selectivity features. With the detailed analysis of the modes, field, and surface current distributions, the mechanisms of wideband, high selectivity, and polarization conversion are revealed. Finally, the linear polarization converter as well as the bifunctional metasurface are fabricated for demonstration. The linear polarization converter can attain a conversion bandwidth of 45% under a polarization conversion ratio of >90%. Due to the excellent filtering characteristic of the proposed linear polarization converter, the constructed bifunctional metasurface can reduce its isolation band to only 2.7%. As expected, the proposed converter has the attractive advantages of wideband operation and high selectivity, which are of great significance for reducing the isolation-band and improving the spectral utilization in various applications of multifunctional metasurfaces.