The interaction and the perturbation between a metal-coated fiber probe and an aperture evanescent field is analyzed numerically using finite-difference time-domain (FDTD) method. The simulation model consists of a sub-wavelength aperture and a metal-coated fiber probe scanning in constant height mode. Influences of the scanning height and the aperture size of the fiber probe on optical field detection are discussed. Relationship between the measured optical field and the initial optical field without perturbation is compared and analyzed. The research results reveal that when the aperture size of the probe is comparable with the measured nano-structure and the scanning height is close to half of the aperture size, the measured field distribution shows better similarity with the initial one, that is to say, it can describe the initial one accurately.