The terahertz polarization converter has potential applications in the field of terahertz spectroscopy and imaging. A broadband and high conversion rate of terahertz linear polarization converter based on the metasurface of resonant ring is proposed. The designed structure consists of three layers, i.e., the top layer, which is a metasurface of resonant ring; the bottom layer, which is a metal film of aluminum; a dielectric layer of polyethylene terephthalate, which is sandwiched in between. In order to obtain the best performance, the simulation and optimization are performed by using CST microwave studio. At the same time, the preparation conditions are also taken into account. The optimized geometric parameters of device are obtained. The samples are prepared by using the photolithography and wet etching. The performance of the designed polarization converter is demonstrated experimentally by using the terahertz time domain spectroscopy. The experimental results show that the proposed device can rotate 90° the polarization state of incident terahertz wave of linear polarization in a frequency range from 0.59 THz to 1.24 THz. The polarization conversion rate is more than 80%. The experimental result is in good agreement with the simulated one. By calculating the polarization angle and elliptical angle of the reflected terahertz wave, it is proved that this device can achieve a high-efficiency linear polarization conversion in a wide frequency range. The distributions of surface currents and electric fields are simulated at the frequency with the high polarization conversion rate. The mechanism of high polarization conversion rate is analyzed based on the distribution of surface currents. The performances of broadband and high conversion rate of the designed structure are derived from the third-order electromagnetic resonance. At the same time, the dependence of the polarization conversion rate on incident angle and polarization angle is stimulated and analyzed. The results show that this device has a good polarization conversion performance in an incidence angle range of 0°-30° and a polarization angle range of-10°-10°.