The basic principle and the sensing structure of a novel torsion sensor are proposed in this paper. The theoretical formulas of torsional angle sensing are derived and the torsional angle of this sensor is linear to the power loss of the multi-mode fiber. In the range of the torsional angle between - 40°and +40°, the torsion sensing has been realized technologically with the sensitivities 0.375 dB/degree for the symmetrically bidirectional linear torsion sensing and 0.592 dB/degree for the bidirectional unsymmetrical linear torsion sensing.