Two kinds of assembly and welding methods of three-stage nuclear power project CV bottom head are introduced,compare and analyze the two methods from the demand amount for auxiliary fittings and the install removal mode,welding space and welding method,the amount of cutting,etc. The result shows that adopt the Backward Assembly of bottom head,the amount of bracket used for assembly will be reduced greatly,which will be help to remove and transport,and the amount of cutting is reduced,in addition with large welding space,which benefit to automatic welding. This paper also analyzes the stress shrinkage and deformation control of weld seam,hoisting and transportating mode,the advantages and disadvantages of the two assembly and welding methods are explored,so as to provide reference for the follow-up similar modules assembly and welding,convenient for the construction company to make full use of its advantages and choose the best assembling and welding method,which can reduce the workload on the basis of ensuring quality.