The study investigates the impact of a modified shield slag backfill top plate on the structural performance of an assembled subway station. Using a three-dimensional model of Huaxia Station from the second phase of Shenzhen Railway Transit Line 6 extension, the analysis compares the force and deformation of station members under identical geological conditions but different backfill slags. Findings indicate that slags with strengths of 12.5 MPa or higher exhibit lower stresses on the structure. Backfill near this strength significantly reduces vertical and lateral deflections of the station structure. Compared to plain fill backfill, maximum principal stresses at the top slab joint decrease by 11% to 54.1%, minimum principal stresses by 26.3% to 71.5%, and maximum Mises stresses by 9.00% to 29.9% upon completion of construction. These results offer insights for constructing prefabricated subway stations.