Abstract: According to the technics of structure member strengthened with R.C,it is designed that two pieces of composite beams by shaped-steel underpinning(CBSSU) jointed with welded short reinforcing steel bars and one R.C beam in order to contrast their experimentation and analysis.The short period loading test by indoor model is used,and it is observed that several parameters of these beam behaviors,such as limit loadbearing capacity,bending stiffness,flaw condition and stress-strain in the middle section of the beam.On the basis of the experimental study,it can be found that the shaped-steel works in coordination with the R.C beam very well.The CBSSU jointed by welded short reinforcing steel bar gains better bending stiffness,higher limit load-bearing capacity and flaw of the beam can be controlled effectively.