It was found that during a survey that part of a mid inner purlin in Zhonghe Palace in the Forbidden City ruptured at the intersection between the purlin and the beam above it, which led to large deflection of the purlin and the tie beam under it. By theoretical calculation and finite element simulation methods, the cause of the purlin rupture was studied. Based on the boundary characteristics of the tenonmortise connection, analytical models of the purlin and tiebeam were built. Situations where the purlin superposes on the tie beam, detaches from the tie beam, and becomes completely ruptured were studied. By static analysis, deflection and inner force distributions of the purlin and the tie beam were obtained, and factors causing the rupture of the purlin were discussed. The results showed that weakening of the bearing section of the purlin at the intersection location caused insufficient bearing capacity of the purlin, which leads to its rupture. Compared to the case where the purlin superposes on the tie beam, the case where the purlin detaches from the tie beam makes the rupture more severe. When the purlin completely ruptures, the upper loads transferred to the tie beam increases sharply, which threatens the safety of the tie beam. Thus it is important to strengthen the purlin in time.