An optic fiber refractive index (RI) sensor based on SMF-MMF-TMCF-SMF (SMTmS) structure is proposed. The MMF segment is introduced and confirmed theoretically and experimentally to act as an efficient optical coupler to launch the light from the center core to the cladding. Experimental results demonstrate that SMTmS is highly sensitive (1435.76 nm/RIU) to low surrounding RI in the range of 1.3330–1.3451. The temperature sensing has been implemented and the total wavelength shift of only 1.83 nm is achieved for the range of 30 °C–100 °C, implying a relatively low temperature cross-sensitivity of 2.01 x 10−5 RIU/°C. The developed sensor with high RI sensitivity and low temperature cross-sensitivity could be rather suitable and applicable in the fields of seawater sensing and environmental monitoring.