Nowadays, terahertz(THz) imaging is an important research focus among the development of THz techniques. And imaging systems based on array detectors which have real time imaging capability have attracted lots of research interests. It is essential to reduce power losses during the system design. Owing to the low energy losses and design convenience it brings, off-axis parabolic mirrors have been frequently used in many THz imaging systems. A 2D THz array imaging system comprised of off-axis parabolic mirrors is analyzed using an optical design software ZEMAX. The deterioration of the image quality caused by the misalignment of off-axis parabolic mirror in the receiving part is investigated. The simulation results show that the image resolution can reach 0.5 mm for the ideal case, and the grid distortion is 0.069%. The system has a tolerance of 1.1° for the misalignment angle of the off-axis parabolic mirror. The tolerance for the decenter error is 6.5 mm.