To analysis the effect of the level of thick plate transition layer on structural seismic performance，according to the code for seismic design of buildings， we systematically studied the failure mechanism and calculation method of structure under earthquake action and built the motion equation of buildings. Natural period of structure，floor displacement and inter-story displacement angle were used as the seismic performance indicators and they were analyzed by mode-superposition response spectrum method.It is shown that with the increase of the thick plate transition layer，the natural period of structure in the low order mode increases，the floor displacement and inter-story displacement angle rise, and the whole structure seismic performance declines.