Deformation mechanism in flow forming of thin-walled tube with helical grid-stiffened ribs was studied based on the combination of numerical simulation and experimental method. The finite element（FE） model for flow forming of thin-walled tube with helical grid-stiffened ribs was first established and verified by flow forming experiment. Then， the stress distribution when the roller rotates at different regions and strain distribution of formed tube were analyzed based on the developed FE model. The results show that the material in cylinder wall region is under three-dimensional compressive stress state. The material in outer surface of rib region is also under three-dimensional compressive stress state， while the material in inner surface of rib region is under compressive stress in the radial direction and tensile stress in the tangential and axial directions. The spun tube is characterized by non-uniform deformation. The plastic strain in outer surface is larger than that in inner surface， and plastic strain in cylinder wall region is larger than that in rib region. Moreover， the strain characteristics in cylinder wall region are compression in the radial direction and tension in the tangential and axial directions. While， the strain characteristics in rib region are radial compression in the outer surface and radial tension in the inner surface.