Nowadays, there are no techniques or software making it possible to simulate the high strains of shells under the combined complex loadings of internal pressure and torsion in the generalized axisymmetrical formulation. These problems are inherent for the experimental investigations of elastoplastic properties of metals and alloys on tubular samples. In this paper, we present the variation-difference method for solving similar problems with taking into consideration high strains and the conditions of an inhomogeneous stressstrain state. We conduct the numerical and experimental studies of the process of elastoplastic deformation and plastic strain localization under the combined loadings of internal pressure and torsion. The mutual influence of torsion on the shell forming under high strains is estimated. The loss of stability with axisymmetric and non-axisymmetric forms is analyzed by 2D and 3D modeling. The simulations results are finally validated by experiments.