The total internal reflection and the system of photon scanning tunneling microscope(PSTM) are simulated using finite difference time domain(FDTD) method. The basic principle of photon scanning tunneling microscope is that the surface evanescent wave generated under total internal reflection condition and the surface evanescent wave is explored. The incident wave is set throughout the problem space in photon scanning tunneling microscope into three-wave included the incident wave, reflective wave and transmitted evanescent wave. The intensity distribution of near field of sphere samples is calculated. And the scanning images can be obtained when the probe of sphere scans along a line above the sphere samples. The results show that the finite difference time domain method is an efficient tool for photon scanning tunneling microscope.