Four unidirectional prestressed space frames set at the critical height (24 m) and the critical span (18 m) were designed according to the Chinese codes, including two frames of the second, third seismic grade in intensity-category seven and two frames of the first, second seismic grade in intensity-category eight ("strong column-weak beam" in the middle nodes of the first seismic grade frame was weakened by the suggested method), based on the OpenSEES framework, elastoplastic time history analysis method was used to analyze the structural seismic performance and capacity under the bi-directional seismic waves. The results showed the yielding degree of every frames in both directions was acceptable and the global seismic capability was good. Besides, the frames which took a lower seismic grade had a slightly larger global seismic response. but the difference was not obvious. An ideal "beam hinge mechanism" was formed in the direction of RC frames and a "column hinge mechanism" was formed in the direction of PC frames, the lateral stiffness and seismic capability of PC frames were better than RC frames'. The PC frame of the first seismic grade formed hinges in middle columns effectively after weakening "Strong Column-Weak Beam", but hinges in side columns were still unavoidable, thus it was considerable to strengthen side columns.