To meet the needs of the multi-function and the multi-use of the modern tall buildings, the megastructure systems have been gradually emerged and used in tall buildings in many countries. In this paper, the analytical model and relative analysis method due to vertical seismic action of the megastructure are presented, including the spectrum and time-history analysis. The results of the calculation of an example show that the vertical seismic action on megastructure is remarkable. It reminds us that the vertical seismic action on megastructures can't be neglected in the design of this kind of structures. Furthermore, it is pointed out that the "story model" is not suitable to megastructures in dynamic analysis.