Depth image of an object can be obtained directly with optical coherence tomography（OCT）. In this paper

a theoretical study on depth image information of the tissue structures by using OCT system was reported. Based on this study

the coherent transfer function（CTF）is derived and the depth-discrimination property is analyzed. In this system

structures with high spatial frequency are detected for image information

and low frequency in the image are contributed by nonlinear processing. This research will provide theoretical basis in quantiatively understanding the OCT measurements of an unknown structure.