Abstract: A bandwidth adjustable terahertz (THz) polarization converter based on vanadium dioxide (VO₂) metamaterial was designed. Considering the working frequency, excitation mode and micromachining process, the performance of the device was designed and simulated by using finite integral technology. The unit structure of the device is a 5-layer periodic array structure with a period of 100 μm in x and y directions. From top to bottom, there are metal oblique-split-ring-resonator (OSRR), first polyimide (PI) isolation layer, VO₂ film, second PI isolation layer and metal base plate. The simulation results show that when the VO₂ is insulating, broadband cross polarization conversion effect can be realized in the frequency range of 0.8 THz, and the polarization conversion rate (PCR) is higher than 90%; under thermal stimulus, VO₂ changes from insulating state to metallic state, and narrowband cross polarization conversion can be realized near two resonant frequencies, and the PCR is close to 100%. By analyzing the surface current distribution, it is found that VO₂ film phase transition changes the physical mechanism of the device, realising the polarization conversion function with adjustable bandwidth successfully. The design method proposed provides a new idea for bandwidth regulation of reflective polarization conversion devices.