Abstract: The pullout of a tenon from the mortise which is one of the typical degradation types of ancient timber structures has significant influence on the seismic performance of the joints and structures. Straight mortise-tenon joints were selected as the object. The load transfer mechanism of such joints with and without pulled-out tenons was analyzed, and the same mechanical mechanism was found. A finite element model (FEM) used to simulate the joint without pulled-out tenons was established. The moment-rotation hysteretic curves from both FEM simulation and the results of a previous test by the authors were compared and they agreed well. The simulation results show that the strength and deformation of a joint with a pulled-out tenon decreases with the increase of the pulled-out length. The FEM and related analyses can provide good reference for the seismic analysis of ancient timber structures.