To explore the improvement mechanism of welding round steel at soffit on the flexural performance of eccentric concrete-filled steel tube (CFST) members, we established a numerical model of CFST beams reinforced with round steel by using ABAQUS software and verified the model by test results. By analyzing the bending momentdeflection curve, bending momentaxial strain curve, hoop strain curve, restraint index, and neutral axis offset of eccentric CFST members reinforced with round steel, the improvement mechanism of the flexural performance of the eccentric CFST members was revealed. Besides, the influence of the diameter of the round steel and the slenderness ratio of the beams on the flexural performance of eccentric CFST members reinforced with round steel was analyzed. Results show that welding round steel could lower the position of the neutral axis of the section and increase the hoop strain of the steel tube on the compression side. Therefore, the concrete area in compression was increased, and the restraint effect of the steel tube on the compression side on the concrete was enhanced. Furthermore, the flexural bearing capacity and flexural stiffness of the eccentric CFST members were improved, and the larger the diameter of the round steel, the greater the improvement. The ultimate bending moment of the eccentric CFST members decreased with the increase in the axial compression ratio, and the larger the diameter of round steel and the slenderness ratio of beams, the greater the reduction. Welding round steel had a better effect on improving the bending performance of the eccentric member with a large slenderness ratio, and the larger the axial compression ratio, the better the improvement effect.