An optical fiber torsion sensor based on a pre-twisted polarization-maintaining fiber (PMF) Sagnac loop mirror structure is proposed and experimentally demonstrated for simultaneously measuring rotational angle and direction. The sensing part of the structure is composed of a 2-cm pre-twisted Panda-type PMF spliced between two segments of 2-mm length multimode fiber (MMF). Torsion direction discrimination and torsion angle measurement can be realized by measuring the wavelength and power of a resonant dip in transmission spectra. The experimental results show that the sensor has a high sensitivity, and the resonant dip shifts with changes in torsion angle and direction. The wavelength of the resonant dip blue-shifts as the torsion angle increases, with a maximum wavelength sensitivity of -455 pm·rad^-1·m and a power sensitivity of -1.35 dB·rad^-1·m for counterclockwise torsion. For clockwise torsion, the wavelength of the resonant dip red-shifts as the torsion angle increases, with a maximum wavelength sensitivity of 182 pm·rad^-1·m and a power sensitivity of 2.20 dB·rad^-1·m. The sensor has the advantages of simple structure, easy fabrication, low cost, and potential applications in the field of torsion or rotation measurement.