Abstract: A solid waste-based ternary cementitious system was formulated using slag micropowder, fly ash, and desulfurization gypsum, which was then combined with cement, polystyrene particles, and a foaming agent. An orthogonal experimental design was employed to prepare integrated composite wall panel materials for prefabricated steel structure residential buildings. The compressive strength, dry volume density, thermal conductivity, and water absorption of the materials under different factor-level combinations were tested. With the compressive strength and dry volume density as the main objectives, the optimal mix proportion for the composite wallboard material was selected. Based on the optimal mix proportion, wall panels reinforced with either steel bars or GFRP bars were fabricated, including specimens with and without window openings. Flexural performance tests were conducted, and parameters such as cracking load, ultimate failure load, concrete load–strain curve, reinforcement load–strain curve, and load–deflection curve were analyzed to evaluate the flexural performance of the materials. The results show that the optimal material mix proportion is a water-binder ratio of 0.4, a terpolymer cementation ratio of 1∶1∶3, a replacement cement ratio of 50%, and a polystyrene particle content of 35% (by volume fraction). At this mix proportion, the wallboard material exhibits excellent comprehensive performance in terms of mechanical properties, lightweight characteristics, and thermal insulation, with a 28-day compressive strength of 11.12 MPa, a dry bulk density of 973 kg/m³, a thermal conductivity of 0.187 W/(m•K), and a water absorption of 10.06%. The wallboard material demonstrates good flexural performance, with steel-reinforced wallboards showing higher cracking and ultimate failure loads compared to GFRP-reinforced wallboards. The presence of openings affects both the cracking and ultimate failure loads of the wallboards.