The optical fiber Surface Plasma Resonance (SPR) sensor is usually based on the fiber core as the resonance substrate. It is necessary to remove the optical fiber cladding by adopting complex processing technologies such as etching, side polishing, grinding, etc. However, there are problems such as evanescent waves that are not easy to leak or sensor probe making them difficult to observe. In this study, a step-index multimode fiber clad SPR sensor with fiber cladding as the SPR resonant substrate is proposed. The transmitted light of a single-mode fiber is injected into step-index multimode fiber cladding by core-shift welding and the 50 nm gold film is plated on the step-index multimode fiber cladding. In the probe sensing section, all the light field energy is distributed in the step-index multimode fiber cladding, the SPR effect is sufficient, and a deeper resonance valley is obtained than by using the conventional fiber cladding SPR sensing structure. When the refractive index measurement range is 1.333—1.385 RIU, the average sensitivity of the sensor can reach 2 307 nm/RIU. This study also explores the influence of different parameters on the diameter and length of the multimode fiber core in the sensing section. The step-index multimode fiber cladding SPR sensor proposed in this study is simple to fabricate and can effectively solve the evanescent wave problem making information difficult to obtain between fiber cladding and air interface.