Containment vessel bottom head（CVBH）,assembled and welded on nuclear power project sites,is a petal-shaped ellipsoidal structure featured by large size,thick walls,multiple curvatures,and high precision,which is adopted by China’s nuclear power industry for the first time. The steel plate formed by special equipment through compression molding on the basis of different radii of curvature has large residual stress. Besides,the number of steel plates constituting each ring is different. These characteristics lead to different welding shrinkage and deformation for each ring and make it difficult to reserve a proper welding shrinkage gap. All these increase the difficulty of the assembling and greatly affect the control of the overall dimensions of the CVBH. Therefore,suitable assembly methods and welding deformation control become an important part of the construction process. This paper studies the two types of assembly,welding sequence,anti-deformation welding fixtures,welding processes and other measures of CVBH in three types of nuclear power reactors. Through such introduction and analysis,this paper has validated that the two types of assembly are both feasible and proposed the direction of improvement for the top-down assembly method.