The invention relates to an additive used in the construction industry, in particular to an additive for a lightweight member in a cast-in-place hollow building roof, which comprises the following components by weight percentage in modifying agent: 5.4 to 34.6 percent of early strength agent, 3.3 to 39.6 percent of diffusion agent, 5.4 to 28.8 percent of water-reducing agent, 2.7 to 21.6 percent of suspending agent, 3.8 to 27.4 percent of foaming agent, and 1.1 to 25.9 percent of foam stabilizer. The modifying agent adopting the proposal improves strength of a member greatly, and the member is quickly molded; the cement foaming agent in the additive has strong dispersion property and does not generate phenomenon such as caking and flocculation, shrinkage, bleeding and deposition during production of foam concrete; the foam concrete has stable air holes, and can be reacted with cement to generate cement solid foam concrete which has low water absorption rate; and the air holes are separately closed, and do not absorb water or store water. The added early strength agent can accelerate settling time of the cement concrete, can meet the characteristics of the concrete such as large fluidity, delayed coagulation, early strength and no backward shrinkage on later strength, has good durability, improves construction efficiency, and can improve the strength of the concrete by more than 40 percent in two days. By using the materials for the modifying agent added according to a proportion, the additive has the advantages of light weight, heat preservation, thermal insulation, large strength, quick formation and the like, is tighter and firmer to joint with other building materials, and can be completely used in the building roof.