Abstract: The typical mortise-tenon joints in historic timber buildings were analyzed in mechanics. The low cyclic reversed loading tests were carried out. The semi-rigid characteristic of the joggle joint and the change rules of stiffness degradation were obtained. The moment-deflection angle hysteretic curves were tested. By simulating the state of mortise-tenon joint with the varied stiffness element, the relation between the varied stiffness and the relative flexibility was found. It is shown that the stiffness change of the mortise-tenon joint is nonlinear from 0.3062 to 23.6054 with the change of load. It may provide information, suggestions and theoretical base for seismic research, protection and maintenance of historic timber buildings.