In order to solve the overall lifting difficulty of mid-span steel beam in the steel-concrete hybrid continuous box girder bridge with variable sections under special environmental conditions, and to provide guidance for the construction and design of such bridges. This study analyzed the external measures adopted by different construction methods and their impacts on the bridge-building state of the structure. It also studied key factors and control points for sub-segment cantilever assembly method in both theoretical research and real bridge calculations. The results indicate that the effect of the temporary load and the temporary external prestressing will not be offset by the demolition after the system transformation, and will affect the structure in the bridge-building state. Additionally, it is found that the sub-segment cantilever assembly method can achieve the bridge bending moment corresponding to the overall lifting method by construction measures, and the external prestressing at the lower part can significantly improve closing state adjustment efficiency compared to using it at the upper part, leading to a substantial reduction in axial force on the steel beam. Furthermore, crane weight has a greater impact on internal forces in sub-segment cantilever assembly method than in whole lifting method. This study provides valuable insights for process design guidance in constructing this type of bridge, including proposing a temporary articulation device at closing ports to mitigate adverse influences from bridge floor cranes on bending moments at the steel beam root.