To aim at the third level seismic fortification target： not collapse in severe earthquake conditions, an optimal design method for spacial frame structures under the severe earthquake loading is perfrothed. According to the design concept： get the best design with the same cost, the optimization mathematics model is established to minimize the maximal relative displacement between neighbor floors in building structure under severe earthquake loading, meanwhile, to satisfy the structural volume constraint. The model is solved with the improved complex method. At last, the 3-D seismic optimal design of a space frame is demonstrated by the second exploitation based on the explicit dynamic analysis soft ANSYS/LS - DYNA. The numerical value result indicates the design method not always achieve the optimum design of the seismic structures, but it can obtain a series of better seismic designs. So the optimal design method can be used in the practical engineering.