A single-optic-fiber sensor is proposed to simultaneously detect the refractive index (RI) and temperature (T) at a single wavelength band. This sensor is based on the mixed effects of Mach-Zehnder interference (MZI) and surface plasmon resonance (SPR), where MZI is excited by a tapered-fiber structure, and SPR is stimulated by a 45 nm gold film on the tapered-fiber surface. The detection signal of an SPR spectrum superimposed on interference stripes was obtained. After fast Fourier transform and filter processing, the MZI and SPR signals were separated. Experimental results indicate that our sensor can improve the RI sensitivity to 2021.07 nm/RIU (21-fold greater than that of the original tapered-fiber MZI sensor) and detect T simultaneously. Additionally, this highly integrated sensor simplifies the detection system, with potential applications in portable biochemical sensing.