Abstract: Taking the circumferential displacement at outlet of deformation in long-mandrel continuous tube-rolling as a parameter to be determined, the author derived the optimum solution of the displacement according to the minimum energy principle by applying the strip-element method and then further determined the plastic flow speed field of the deformation zone. The strip-element method, due to such features as accurate calculation, low cost and simplified data-input, surely becomes a new efficient calculation method for theoretical plastic analysis in long-mandrel continuous tube-rolling.