To control the deformation difference between the to-be-installed and installed segments during the cantilever assembly of wide steel-UHPC composite girders in cable-stayed bridges and achieve precise matching, this paper first introduced a matching method combining pre-hanging cables with crane unloading. Then, by taking the Guanyin Temple Yangtze River Bridge (a super-kilometer-span cable-stayed bridge with wide steel-UHPC composite girders) as the engineering background, a locally refined finite element model was established to analyze the vertical deformation laws of the matching sections during the cantilever assembly construction phase. Finally, the effects of matching sequence, pre-hanging cable force, and crane unloading ratio on the deformation and stress of the girder segments were discussed. Calculation methods for pre-tensioning cable force and unloading ratio were proposed, and the optimal unloading scheme was determined. The results show that the method of pre-hanging cables with crane unloading can effectively reduce the vertical deformation difference between the to-be-installed and installed segments during the cantilever assembly of wide steel-UHPC composite girders, creating favorable conditions for the precise matching of the splicing between to-be-installed and installed segments.