Abstract: According to the priinciple of elastic-plastic bending deformation, this paper has analyzed the bending deformation process in the longittudinal seam welding tube forming machine, and thus, has proposed a calculation methodfor determining the parameter of the load and the moment. And by applying the physical model of the infinitive length tube submitted to locally uniformly distributed external compression, the darived calculation method for deter mining the sized tube gage, the load and the momeut with consideration of the sizing tube gage decremnt and the elastie restoration will be obtained. Finally,taking account of the static and dynamic load and considering what the electrical control system is, we can satisfactorily select out the required motor.