Abstract: The seismic performance of steel frames with reduced beam section connections is evaluated by using the finite element analysis, the modal analysis and the time-history analysis. The vibration period, the dynamic internal force, the dynamic stress and the plastic deformation zone in steel frames are studied in detail; and the results are compared with those of steel frames with ordinary connections. It is shown that the seismic performances of the steel frames with reduced beam section connections are not very satisfactory under weak earthquakes, but they are very good under strong earthquakes, the plastic deformation occurs at the reduced beam, the plastic strain increases, the column base shear and the top displacement of the steel frames are all smaller than those of the steel frames with ordinary connections, so it is suggested that reduced beam section connections should be used in areas with strong earthquakes.