A new concept of“double substitute structure”was established, combining the new concept with the basic iterative process, a new direct displacement based seismic design method considering global reliability was proposed.This new iterative design method use“relative reliability theory”to determine the displacement target corresponding to the given performance objectives, and transform the displacement target into corresponding base shear target precisely by“double substitute structure”, then calculate the exact designing base shear force to finish the iterative seismic design process by introducing a correction factor. The calculation examples showed that the new direct displacement based seismic design method can accurately control the displacement response of structure in the sense of global reliability.