This paper demonstrates a dual-functional terahertz metamaterial that utilizes the phase transition properties of vanadium dioxide (VO2). By altering the conductivity of VO2, the role of the proposed metamaterial changes from absorption to reflective polarization conversion. The unit cell is the stacked structure with different VO2 patterns, polyimide-slabs (PI), gold pattern and gold plate. When VO2 is metallic state, the proposed metamaterial demonstrates ultra-broadband absorption within the 5.8–17.5 THz range, achieving absorption rate superior than 90%. In the non-metallic state of VO2, The metamaterial displays outstanding performance in converting linear-to-linear polarization. This performance is observed in the range of 6.1–20.9 THz, where the polarization conversion rate exceeds 90%. The operating principle of the metamaterial is clarified by analyzing the electric field in the absorption state and the surface currents during the polarization conversion state. Furthermore, the ultra-broadband absorption of the metamaterial is explained using impedance matching theory and multiple interference theory. Mechanisms behind polarization conversion are clarified through the use of Jones vectors and the Jones matrix. Theoretical calculations align closely with simulations, validating their accuracy. The proposed metamaterial excels in ultra-broadband absorption and polarization conversion. As a result, it holds promising prospects for applications in terahertz stealth technology, communication systems, radar, and other advanced domains.