The forming device and process of thin-walled tube with variable section along the axial direction were realized by the hydromechanical forming method. The results show that compared with necking, liquid extrusion increases the limited degree of deformation to 26%. Compared with necking and liquid extrusion on the moving mandrel, the deformation degree of various relative deformation thickness S0/D0 for tube blank can be expanded to 33%, and the corresponding σz and p values are optimized in the hydromechanical forming process of thin-walled tube because there is liquid support around blank,and there is no need of filling liquid or filler in the tube, then the driving force is also a non-impact pressure liquid during the hydromechanical forming. Furthermore, in the cycle of hydromechanical forming for tube blank, there is no mandrel or plug device in the tube blank cavity before deformation, and there is no need to extract the mandrel or plug from the cavity after deformation. Thus, the deformation degree of this example reaches 33.3%, and the labor productivity reaches 40-50 pieces·h-1.