Abstract: To enhance the aseismic performance of ductile steel frame structures, an exterior beam-column joint strengthened with a haunch is proposed. Based on the joint's force-transfer and deformation mechanisms, a theoretical formula for calculating the shear transfer coefficient was derived. Quasi-static cyclic loading tests were conducted to systematically analyze the joint's mechanical behavior across four characteristic loading stages, validating the accuracy of the theoretical formula. Further investigated were the influences of the haunch's inclination angle, of the horizontal length ratio, of the cross-sectional area on the shear transfer coefficient, of joint internal forces and, of the haunch stability. The research results indicate that when the inclination angle is 30°-45° or the horizontal length ratio is 0.2-0.3, the shear transfer coefficient falls within the range of 1-3. This configuration significantly reduces the internal force demand in the panel zone. It also ensures the haunch stability and practical spatial feasibility. Increasing the cross-sectional area of the haunch can substantially enhance this optimization effect. These findings provide a theoretical and practical guidance for the aseismic design of haunch-strengthened steel frame connections.