This paper proposes an innovative dual-function damper based on the elastoplastic torsional deformation of steel tubes. The damper adopts a V-shaped polyline design， enabling the components to achieve elastoplastic torsional deformation under external loads， thereby providing both load-bearing and energy-dissipation capabilities. Theoretical analyses were conducted to derive fundamental mechanical parameters， including the initial stiffness， yield load， and yield displacement of the damper. A dual-torsion tube three-fold V-shaped brace， referred to as the double-torsional-tube V-shaped brace （DTTB）， was designed and utilized as a diagonal brace for frame structures. Under quasi-static low-cycle loading conditions， experimental and simulation results revealed that the damper exhibited full hysteresis loops and strong energy dissipation capacity. The equivalent viscous damping coefficient reached 0.38 at the design displacement of 50 mm， meeting the performance level of similar buckling-restrained brace （BRB）. The stiffness degradation and energy dissipation characteristics of the specimens were found to be similar to those of BRBs， validating the feasibility of the proposed damper as a dual-function energy-dissipative brace. The simulation results were highly consistent with experimental observations， verifying the accuracy of the theoretical model. By optimizing the initial angle of the support bar to 25°， the axial tension-compression imbalance coefficient was reduced to 1.2， meeting the requirements of relevant standards and further verifying the engineering feasibility of the design. Finally， the issue of excessive axial tension-compression imbalance caused by geometric nonlinearity was analyzed in detail， providing theoretical insights for enhancing future design schemes.