In this study, an obliquely coupled symmetrical five-cylinder metasurface structure, which can realize a broadband terahertz reflective polarization converter, is proposed. By using the finite difference time domain (FDTD) method to numerically simulate the structure, the results show that the structure can realize cross-conversion between linear polarizations in the frequency range of 2.35?4.51 THz, and the polarization conversion rate exceeds 99%. When changing the long axis parameters of the middle cylinder, the conversion between linear polarization and circular polarization can also be realized in the range of 2.42?4.83 THz, and the reflection coefficient in the conversion frequency range is higher than 0.9. The structure designed in this study can realize the switching between half wave plate and quarter wave plate by adjusting the size of the long axis parameter of the middle cylinder, and it has excellent performance, which leads to certain application prospects in the field of polarization modulation multifunctional devices.