Analytical expressions for scalar/vector angular spectrum transfer function (scalar-/vector-ASTF) of photon scanning tunneling microscope (PSTM) were derived using Fourier angular spectrum diffraction theorem. The numerical results show that the ASTF of a PSTM consists of two distinct components, the propagating component and evanescent component. For evanescent component, their amplitudes decrease dramatically, and the higher the frequency the faster the amplitudes decreasing, while the phases remain constant as tip-sample distance d increases. For propagating component, their phases increase linearly but not uniformly with d, and the lower the frequency the faster the phases increasing, while the amplitudes remain constant. It is the ability of a PSTM to collect evanescent component that makes it possible to break through the Rayleigh diffraction limit. Based on ASTF the influence of such parameters as tip-sample distance, tip aperture diameter and the incident angle of illuminating laser beam on PSTM image has been discussed.