In order to satisfy the requirements of terahertz time-domain spectrum system under specific circumstances, an off-axis focus reflective polarization converter in terahertz band is proposed. By combining the principle of phase compensation and phase gradient metasurface, a reflective array containing units is designed. The phase distribution along the metasurface is calculated through the principle of optical path reversibility. Geometric rotation and resonant frequency modulation constitute the phase variation of the unit, which can be superimposed on each other without interference. Compared with the conventional reflective polarization converter in terahertz band, the proposed one could deflect the normally incident terahertz wave while providing larger energy at the focus. The simulation results show that the proposed polarization converter has good performance in both polarization conversion and electromagnetic focus, which has significant practical application in numerous situations.