To shorten the duration of construction and reduce the costs, identifying the number of floors supported by scaffolds is necessary to construct a multi-story building with the floors plate of long-span open-web sandwich slab, composed of the upper and lower ribs, shear key, and surface sheet. This study proposes a favorable method of two-stage construction support floors, namely the construction stages I and II, by establishing their finite element models that consider the effect of the horizontal tube constraining the upright tube in scaffolds. Furthermore, to ensure accuracy of the model in construction stage II, the full-hall supports of the first floor in construction stage I are equalized to uniform surface loads, and then the two construction processes are simulated by considering the upright tubes of the scaffolds and the structure. Comparison of the two modeling results with field-measured data shows that the equivalent methods of the upright tube and the full-hall supports are feasible. In addition, for an open-web sandwich floor slab whose span is ≤24.00m, only the full-hall supports of the floor can be retained if its lower floor is the basement roof; otherwise, the full-hall supports should be retained both on this floor and its lower one. When the span of the floor slab is 39.00m and if the span of the lower two floors slab is ≤24.00m, reaching the concrete design strength, then the full-hall supports should be retained on this floor and the lower floor, and partial re-supports should be applied to the second lower floor.