Abstract: 2219 aluminum alloy is widely used in the tanks of a new generation of launch vehicles. And friction stir welding technology is widely used in tank welding because of its high welding performance. However, the roundness of friction stir welding seam is not always satisfactory, and the local cracking of the thermal insulation layer will occur. That does not meet the requirements of low temperature insulation, and even leads to the failure of launch vehicles’ flight test. Therefore, it is necessary to carry out the analysis and research on the roundness of friction stir welding seam in the tank shell of the new generation of launch vehicles. In this paper, the influence factors of the roundness of the tank shell are analyzed through the tank’s welding and charging tests, and the influence of the roundness of the tank shell on the thermal insulation layer is analyzed through the insulation test. At the same time, the finite element analysis (FEA) software ABAQUS is used to simulate the charging process of tank shell made by 2219 aluminum alloy, and the maximum deformation under different roundness of the tank shell is analyzed. The results show that the roundness of the tank shell panel before welding and the characteristics of friction stir welding process lead to the unsatisfactory roundness of the friction stir welding seam after welding. During repeated pressurization in the assembly and testing period, the positions of inferior roundness will have larger deformation, which can lead to the local cracking of the thermal insulation layer because of exceeding self elongation.