In this paper, two kinds of mega-frame structures have been simplified to the series-parallel-connection model which considered with joint rotation. The stiffness matrix and mass matrix also have been established and predigested. The vertical stiffness matrix and mass matrix have been obtained. There has been a dynamic characteristics analysis on the structures subjected to vertical seismic force with the method of analysis of step-by step integer. From the results some dynamic characters of structures subjected to vertical seismic force have been obtained.