There are many advantages in steel-concrete structures, such as lighted-dead weight, excellent performances of ductility and seismic resistance, rapid construction speed and significant integrated economic benefits with increasing of the quantity and quality of the steel products and improving of the manufacturing and assembling technique of the steel buildings. In the structural design of composite frames, one of the basic assumptions is that the connections between columns and beams are either perfectly rigid or perfectly hinged. However, in actual structures, typical connections do not behave in either a perfectly rigid or a perfectly hinged manner and the connections have been proved to be semi-rigid practically. However, there is a lack of an overall frame design approach matching it. The purpose of the paper give a calculating method that accords practical engineering and easily put into effect. Worthwhile it is going to promote the development of semi-rigid composite steel frame in design and heighten structural stability.In the paper, the practical design approach for semi-rigid frame under lateral load is studied, and a self-characterized research is conducted in several aspects as following: On the base of comparatively analysis about various design method to beam-to-column connections, some modified computing equations about the initial stiffness , ultimate moment capacities and rotation capacities are presented, and the main factors influencing upon the behavior of connections are summed up.On the base of understand about the characteristics of the members of the composite steel frame, the end fixity factor which reflects connection stiffness is introduced. Structure stiffness matrix and member fixed end moments are modified by considering semi-rigid effect. A simplified approach to calculate the deflection of semi-rigid sway frame under SLS (Serviceability Limited State) is put forward.The plastic analysis approach are concretely brought forward to analyze internal forces of semi-rigid sway frame under lateral loads under ULS (Ultimate Limited State). It shows that moment capacity and ductility of connection should be considered under ULS; plastic analysis approach does not do overall structure analysis, computing time is reduced and the result is quite good.Full-scale testing of the composite frame with semi-rigid composite connections is first carried out in china in order to obtain the behavior of semi-rigid composite connections steel frame under lateral loading as well as to analyze collapse characteristic after structural ultimate strength has lost coming order of plastic hinges.