Two-dimensional image formation in OCT system was analyzed based on the weak object approximation. It was proved that the OCT system behaves as a linear shift inyariance system. The derived 2-D CTF shows that the system performs as a low-pass filter. Smaller distance between the fiber and the lens will not spoil the performance seriously. Decreasing the focus and enlarging the pupil will broaden the system bandwidth, and improve the imaging performance.