To increase the sensitivity of fiber-optic evanescent wave sensors, a sensing optic fiber with novel structure was proposed in this paper. The sensing optical fiber was by fabricated by cascading of a traditional plastic optical fiber, a novel step optical fiber (with a refractive index increasing step from the fiber core to the cladding) and another traditional plastic optical fiber together. The new step optical fiber was the sensitive area of the evanescent wave sensor. The fiber core was made of polymethyl methacrylate, and the cladding was made of a mixture of polysulfone and GeO2. The optical transmission characteristics of the prepared optical fiber were analyzed by using geometrical optics. The optical transmission characteristics and its sensitivity of the prepared optical fiber to the external refractive index were investigated. The results highlight that when the GeO2 doping content in the cladding of the new step optical fiber was 1.5 wt% and the coating thickness was 200 μm, the sensing fiber showed good spectral transmission quality and high refractive index response sensitivity.