The image of photon scanning tunnel microscope (PSTM), which uses fiber probe with metallic nanoparticle tip, is analyzed with the finite-difference time-domain (FDTD) formulation of dispersive materials. With considering the interaction between the field of the fiber probe and the sample, the influences of different angle of incidence, different distance between probe and sample, different kinds of metal and different size of the metallic nanoparticle on the resolution and the sensitivity of PSTM are investigated with the equivalent incident wave method. The simulated results show that the image of PSTM with metallic nanoparticle tip has higher resolution and sensitivity. The influences of different kinds of metallic nanoparticle on the resolution are not obvious, while the image of PSTM which uses fiber probe with silvery nanoparticle tip has the best sensitivity.