A terahertz broadband polarization converter based on anisotropic metasurface is proposed, which is composed of metal-dielectric-metal three layers. The top layer is a square-shaped resonator intersecting along a diagonal line. The bottom layer is the metal plate. The top and bottom layers are separated by a dielectric layer. The reflection ratio and polarization conversion rate of the structure in the studied frequency range are calculated by simulation. The results show that the polarization converter can rotate the polarization direction of the linearly polarized terahertz wave 90° in the frequency range of 0.4-1.04 THz, and the conversion rate is more than 90%. Meanwhile, the distribution of the surface current of the structure is simulated at the frequency with high polarization conversion rate, and the mechanism of high polarization conversion rate is analyzed. Compared with the previous design, the polarization converter has a simple structure and a broad operation bandwidth, and has a potential application value in the field of terahertz polarization modulation.