Abstract: The semi-rigid mortise-tenon joints in the wooden structures of ancient buildings are often destroyed before beams and columns under major earthquakes, which are the important positions for reinforcement and protection. A new type of shape memory alloy (SMA) preventive reinforcement device is designed, and low-cyclic loading tests are carried out on five reinforced joints and one unreinforced joint. The failure modes, moment-rotation relationship, strength degradation, stiffness degradation, energy dissipation, self–centering ability and deformation capacity of the joints reinforced by SMA strings with different numbers and different pretension strains are studied. A calculation method for bending capacity of straight tenon joints reinforced by SMA strings is established. The results show that the bending capacity of the joints after SMA reinforcement is improved. The maximum bending capacity of the reinforced joint is 1.49 times that of the unreinforced joint. Compared with the unreinforced joint, the strength degradation of the reinforced joints is not serious, which indicates that the SMA strings can still continue to provide the bending capacity for the joints when the rotation is large. The maximum rotational stiffness of the reinforced joint is 2.24 times that of the unreinforced joint. With the increase of the rotation of joints, the energy dissipation of unreinforced joint gradually decreases, however the energy dissipation of reinforced joints increases first and then decreases. At the same rotation, the relative residual deformation decreases with the increase of numbers and of pretension strains of SMA strings, and the self-centering ability is enhanced. The results of research can provide a reference for the preventive conservation of the mortise-tenon joints in ancient timber structures.