With the consideratioin of both defocused and abaxial cases,the analytical expressions for the 3D transfer function of optical low coherence tomography are derived in this paper.The theoretical stud-ies are concerned with the resolution-related factors,which furnish evidences for the designation and optimization of the systems.According to our study,the axis resolution would increase linearly with the spectral bandwidth,while the signal level would decrease linearly.In the case of defocused scan-ning,although longer focus length and narrower beam aperture could result in an absolute decrement of the resolution,they give an even better defocused imaging property in the whole scanning region.The detected waveform and the actual image of the optical coherence tomography are also given.