In this article, a single mode-D-type-multimode fiber structure is designed and fabricated. The interference among the guided modes and the asymmetrical structure of the D-type fiber can be utilized to obtain the specklegram, which is extremely sensitive to external changes, and the eigenvalues of the specklegram are demodulated based on the texture feature algorithm. The refractive index (RI) measurement could be achieved via the relationship between the eigenvalues and RIs. In the study of the RI sensing characteristics, the effects of different polishing lengths and polishing depths on the sensing characteristics are discussed from the point of view of simulation and experiment. Both the simulation and experimental results show that the D-type optical fiber structure with the parameters of polishing length 2 mm and polishing depth 18 μm has the best refractive index sensing performance. The consistence between the simulation and experiment demonstrates the correction of the research method and the feasibility of the D-type fiber specklegram sensor for RI sensing. The maximum RI sensitivity of the EN reaches -1. 86 / RIU, and the fitting degree reaches 0. 964. The maximum RI sensitivity of the COR reaches -0. 23 / RIU, and the fitting degree reaches 0. 979.