Based on calculating of numerical simulation,the reason why the length of the surface rupture caused by the 1927 Gulang M_S8 earthquake is much short was further interpreted theoretically.Distribution pattern of seismic intensity,impeded processes of surface rupture,decline process of displacement vector,fault deformation and distribution characteristics of the maximum shearing stress were studied.Results show the NNW-trending faults are a kind of "barrier structure" which impeded propagation of the seismic rupture toward east and west,and resulted in a short rupture length.