Abstract: In order to study optical properties of an optical fiber probe in scanning near-field optical microscopy (SNOM), simulation experiment was carried out by using the optical software VirtualLab Fusion based on field tracing method. The distribution of external optical field at the tip of SNOM optical probe was obtained. The results show along z axis, the distributions of optical field on different sections show the pattern of small aperture diffraction, the center intensity of center sample decreases exponentially with the increase of z value, and is almost attenuated to 0 at the position of z=100nm. With the increase of z value, full width at half maximum of center speckle contour remains stable at first and then increases. Its inflection point is at the position of z=20nm. The corresponding center strength value is 7.2V/m² at this point. If calculated by the exponential function, this strength is at exactly e^-2 of the position of z=0nm. The study shows the optical properties of SNOM optical probe. It confirms that it is necessary for SNOM probe to keep the distance of around 10nm from the sample surface.