Taking the on-site assembly and welding of the steel containment vessel of a nuclear power plant as an example,this paper introduces the composition of the steel containment vessel of a nuclear power plant,the welding characteristics of SA738Gr.B material,difficulties in the construction process,welding process,common methods for controlling welding deformation,etc. Combining with the characteristics of the structure of the steel containment vessel,the reasons of welding deformation during assembling and welding of steel containment vessel are analyzed. Taking the assembling and welding of bottom head of steel containment vessel as an example,the measures for controlling the welding deformation of the bottom head are described from the groove design,the gap of the groove group,the amount of misalignment,the tack welding and the welding process and so on,the practical results show that the proposed measures of welding deformation is feasible. It provides a reference for the welding deformation control of steel containment in nuclear power plant.