The mechanical properties of the Ban mortise-tenon joints of the ancient traditional timber buildings in the South Yangtze River Regions were studied with the experimental and theoretical methods. The tested mortise-tenon joints include three Ban mortise-tenon joints. The failure modes, the hysteretic curves, the skeleton curves, the rotation rigidities of this kind of mortise-tenon joint under low cyclic loading were obtained. The results show that the hysteretic curves of the Ban mortise-tenon joints appear to be the Z shape and have the obvious pinch effects. During the process of the test, these mortise-tenon joints orderly pass through the elastic stage, the yield stage and the failure stage. Furthermore, with the consideration of the nonlinear contact influence of the mortise-tenon joint and the nonlinear stress-strain behavior of the timber material, the mechanical properties of the Ban mortise-tenon joints were analyzed by ANSYS software. The theoretical results agree well with the experimental results. The relationship of in-plane rotational rigidity, the out-of-plane rotational rigidity and the tortional rigidity of the Ban mortise-tenon joint is achieved, the ratio of them was 1.4∶1.0∶1.1. The results can provide the theoretical basis for computing analysis and repair design of the traditional timber buildings in the South Yangtze River Regions.