Based on the elastic-plastic forming theory and Finite Element Method (FEM), a numerical model is established to investigate the forming of spatial tubes in spatial tube bending-twisting process. Investigations are focused on three aspects, including the forming principles of spatial tubes under combined bending and twisting, the variations of forming parameters under different eccentricities of fixed die and the non-linear relationship between the eccentricities of fixed die and the torsion adjustment indexes. The conclusions are: (1) In spatial tubes bending-twisting process, the real bending radius can be influenced by the rotation angles of mobile die and clamp. (2) The equivalent bending points acquired from pure bending are not suitable for spatial tube bending, due to the fact that the torsion adjustment index at equivalent bending point can be influenced by other factors (eccentricity of fixed die, clamp rotation speed, etc). (3) If the twisting angle keeps constant, the torsion adjustment index will increase accordingly with the increasing of clamp rotation speed, and decrease with the increasing of eccentricity of fixed die.