A method to improve the refractive index (RI) and temperature sensitivities of optical fiber based on surface plasmon resonance (SPR) sensors is proposed and experimentally demonstrated. It is realized by using a precision femtosecond laser system to manufacture microstructures on a heterocore optical fiber structure (multimode single-mode multimode fiber, MSM). The microstructured MSM structure fiber-optic sensors were coated with 60-nm gold (Au) film to test and verify RI sensing, obtaining an enhancement of the maximum sensitivity range from 2845.18 to 3313.15 nm/RIU. The fabricated sensors were additionally coated with a layer of polydimethylsiloxane, which has a high negative thermos-optic coefficient, to conduct a series of temperature sensing experiments. Experimental results showed that the maximum sensitivity increased from 1.1998 to 1.5646 nm/degrees C. Compared with nonmicrostructured sensors, the RI and temperature sensitivity of the proposed sensor has increased 16.4% and 30.2%, respectively. The simply fabricated, low-cost, and high-sensitivity SPR sensor has promising applications in many areas, especially in the biochemical field. (C) 2018 society of Photo-optical Instrumentation Engineers (SPIE).