To solve the springback and welding deformation problem of case shell, a new sizing method based on rubber fluid forming was proposed. Using a casing as the object of study, the pressure of case shell sizing is preliminarily determined by theoretical calculation. The dynamic explicit finite element method is adopted in the plastic deformation mechanism and circular alignment behavior of thin-walled welded cylindrical parts with the same basic characteristics as the casing is analyzed. And the results of finite element analysis show that the initial diameter, runout and wall thickness of the cylinder have influence on the runout  orrection effect. The calibration effect of rubber fluid forming method was verified by a self-made calibration device.