Abstract: The research on mechanical property and microstructure of architectural steel with high strength and low yield ratio has been done，and the results show that rolled steel meets the technical requirements and has favorable impact property． The strength of Nb steel is slightly lower than the technical requirements，which can meet them after its tempering． The ideal start cooling temperature for Nb steel is 750 ℃ or 800 ℃，that of the rest steels is 750 ℃． There are many precipitated phases homogeneous distribution in Nb + Mo steel that mainly are flocculent and irregular particles and their size is generally smaller than 250 nm． The flocculation phase composition is mainly Nb as well as a spot of Ti，that of particles is mainly Nb and Ti，their sizes are 10 ～ 25 nm and 50 ～ 170 nm respectively．