[Purposes] This study aims to verify the structural reliability of the steel-ultra-high performance concrete (UHPC) joint section and to propose optimization suggestions. [Methods] Using Tongji Bridge, a single-tower hybrid girder cable-stayed bridge with a main span of 200 m, as the engineering case, a finite element model of the whole bridge and a local refined model of the steel-UHPC joint section were established using finite element software. By comparatively analyzing the stress distribution, stiffness matching characteristics, and internal force response of the joint section at different locations, the mechanical performance indicators of the joint section were obtained. Meanwhile, to address the shortcomings of existing research methods, a new method for evaluating the smoothness of stiffness transition in the joint section was proposed, and the stiffness transition smoothness under each scheme was calculated by varying the length of the steel-UHPC joint section. [Findings] The steel-UHPC joint section filled with UHPC has a reasonable stress distribution and high bearing capacity. The length adopted in the original scheme is optimal, while shifting its position 2 m toward the main span can further improve the stress state and enhance cost-effectiveness. [Conclusions] The results obtained using the proposed evaluation method are basically consistent with the actual engineering conditions and have value for further in-depth study.