To make sure the seismic behavior of outer-jacketing mega frame for storey-adding, a low-cyclic loading test of prestressed concrete beam and a pseudo-static test of Mega frame were analyzed by elastic-plastic finite element program IDARC2D, compared with the test results, skeleton hysteretic curves and restoring force models of structural member were determined. They were used in IDARC2D to study the seismic behavior of mega frame for storey-adding. Some structures designed complied with the Code for Seismic Design of Buildings (GB50011-2001) and correlative literatures about collapse, these structures were reanalyzed after enhancing their seismic measures suitably, collapse were avoided. They are: the main frame of outer-jacketing mega frame in the zone of seismic fortification intensity 8 conforming to seismic grade 1-st, the height of mega frame under 50m conforming to seismic grade 2-nd and that over 50m conforming to seismic grade 1-st in zone of seismic fortification intensity 7. Research achievements will provide reference to engineering application of this structural system.