This paper expounds the application of rotary expanding process to manufacture the large-diameter hot-rolled seamless gas cylinder. Through analytic geometry method, an equation is established among the tail roll gap, roll distance and the plug protrusion distance. 3D drawing software CATIA-V5 is applied to build 3d models of steel tube and rolling tools. The rolling process is simulated by MSC.Marc FEA (finite element analysis) software. Marc’s second development function and FORTRAN software’s extracting finite element node coordinates function are applied to calculate the wall thickness uniformity. A novel method to calculate the wall thickness uniformity after finite element analysis is proposed. The wall thickness uniformity of steel tube after rolling is well simulated and compared by MSC.Marc FEA software, which can help the technologist to predict and choose the best rolling parameter. For gas cylinder rolling, the tail roll gap should be set to 17mm. The roller distance and the plug protrusion distance should be set as 342mm and 78mm separately.