The properties of model field and evanescent field are theoretically studied and simulated using finite element method, which have a certain extent effect on the sensitivity of the fiber evanescent field sensors based on eccentric core optical fiber. The distributed sensing length of the fiber evanescent field sensors is also calculated based on the Rayleigh scattering theory. The results show that in order to obtain high sensitivity in evanescent field sensing, the eccentric distance is designed to be about 8~10 μm and the refractive index of fiber coating should be low as soon as possible. The sensing length of the fiber evanescent field sensors is estimated to be 16~160 m.