A sustainable building steel structure system，which is kind of damage‐controlled structure，consists of columns，beams and newly developed connections with buckling‐restrained braces like knee brace．The columns and beams are connected with special long steel bars alone，facilitating demolition of the structure and able to be reused the members．Since the connections of beam‐to‐column are semi‐rigid，the columns and beams suffer no damage．The buckling‐restrained knee braces only absorb seismic energy．In the past studies，full‐scale tests of the connection were carried out．Based on these results，structural models，which modeled the connection were analyzed and a mechanical model for the sustainable building structure system was established．In this paper，a structural design method is proposed on the previous experimental and analytical studies． The detail design method for the newly developed connections with buckling‐restrained braces is established．A trial design is conducted where applicablity of the design method is shown.