In this study, fully-thread countersunk head self-tapping screw (STS) with outer diameter of 6 mm and 8 mm were used to reinforce the mortise-tenon (MT) joints with horizontal and inclined insertion methods. A total of fifteen cyclic loading tests were carried out for one group of unreinforced joints and five groups of reinforced joints, and the load-carrying capacity, strength, stiffness, ductility and energy dissipation of these joints were evaluated. The results showed that larger diameter STS and more STS could improve the load-carrying capacity and initial stiffness of the joints, although it might accelerate the damage to the joints and change the failure modes of MT joints. In addition, different reinforcement methods had different improvement effect, as the joints reinforced by STS horizontal insertion had a more significant improvement in the load-carrying capacity, while the inclined insertion of STS had a more significant improvement in the initial stiffness. This study provides data for reinforcement of MT joints with STS, and different reinforcement methods can be selected according to different engineering requirements.