[Purposes] The mixed steel-concrete composite girder cable-stayed bridge mainly consists of the side-span cast-in-place concrete stiffening girder and the mid-span steel-concrete composite stiffening girder. The stiffness of the side-span stiffening girder is significantly greater than that of the mid-span one. During the system conversion process of the bridge engineering, the change in boundary constraints will have an irreversible impact on the bridge. At present, the theoretical materials available for reference on the mixed steel-concrete composite girder cable-stayed bridge are still incomplete, and there is a lack of case studies. There is still value to be explored. [Methods] To optimize the stress state and geometric shape of the mixed steel-concrete composite girder cable-stayed bridge during the system conversion process, this paper took the Sichuan Yanpingba Yangtze River Bridge as the engineering background, used a large-scale finite element software to establish a high-precision and refined calculation model, and conducted a refinement analysis of the three system-conversion schemes of the mixed steel-concrete composite beam cable-stayed bridge. [Findings] The results show that under the condition that the temporary consolidation between towers and beams and the side-span support are released after the middle span is closed, the corresponding full-bridge system has less stress during the conversion process, the geometric shape is stable, the deflection of the main girder is controllable, and the deflection of the cable tower is reasonable. [Conclusions] The construction plan of first closing the middle span and then releasing temporary constraints is recommended.