With strong evanescent waves, optical microfibers (MFs) provide guided lights the ability to directly interact with surrounding environments, whereby several fiber optics chemical sensors have been realized. In this study, based on MFs external refractive index (RI) sensitivity, a multimode optical fiber (MMF) specklegram RI sensor with MMF–MF–MMF configuration is presented. As the MF Section is exposed to the liquids of different RIs (in the range of 1.333–1.368), the interaction between the liquids and evanescent waves modulates the guidance status of the MF, thereby changes the excited modes within the end MMF Section and affects the output speckle pattern accordingly. The evaluation of the functionalities of the MFs with different waist diameters (11, 18, 33, and ) shows that the MF with 33- waist diameter results in the highest output specklegram RI sensitivity, which has been quantified by the zero-mean normalized cross correlation coefficient (ZNCC). Moreover, the response time and sensitivity of the proposed fiber specklegram sensor (FSS) have been simultaneously improved by applying spacial filter on the captured speckles. The RI sensor has also been studied for the temperature detection and showed 0.013 linear sensitivity within the range of 25 °C–65 °C. Finally, the theoretical analysis of the supported modes by the MFs of the specified waist diameters verifies that 33- sample with high number of guided modes and strong total evanescent waves is the optimum case for the MMF–MF–MMF specklegram RI sensor.