Polarization state of electromagnetic waves plays a significant role in the fields of signal transmission and sensitive measurements. High-efficiently manipulating and controlling polarization state by two-dimensional flat metamaterials over a wider bandwidth has been turned into hot issues in recent years. A polarization conversion metasurface based on the split elliptical ring resonator is designed, simulated, and experimentally validated in the microwave regime. The proposed metasurface can convert a linear polarization state into its orthogonal one with a high efficiency for an ultra-wide band. Theoretically, the mechanism of polarization conversion is explained by the theoretical models of high-impedance surface and multi-plasmonic resonances. The metasurface has a strong anisotropy, which behaves as a high-impedance surface, and serves as a metal sheet in orthogonal orientation in the vicinity of the resonant frequencies. The reflection phase has a delay of π for one of the two electric field components and remains unchanged for the other. As a result, the polarization angle of the synthesized reflection electric field rotates by π/2. The fourth-order plasmonic resonances are generated by the electric and magnetic resonances, which contribute to the bandwidth expansion of cross-polarization reflection. Numerically, by means of simulation and analysis on the axial ratio and flare angle of the split elliptical ring resonators, the influences of these structure parameters on the bandwidth and efficiency of the polarization conversion are clarified. And then the design method of multi-peaks and wideband polarization conversion metasurfaces with split elliptic ring resonators is proposed for different kinds of applications. Experimentally, the geometry is implemented within the currently available printing circuit techniques, and a free space method is adopted to measure the scattering coefficients. A polarization conversion ratio of the fabricated sample is larger than 85% at a relative bandwidth of 104.5%, and approximately 100% of the polarization conversion ratio can be achieved around the resonant frequencies. Experimental results are in good consistency with the simulation results. Compared with the anterior polarization conversion metasurfaces, the proposed metasurface broadens the cross-polarization bandwidth greatly with little efficiency expenses. These works provide beneficial guidance for manipulating and controlling polarization states of electromagnetic waves, and have potential applications in modern radar and communication systems, signal detection systems, and sensitivity measurement systems, etc.