Abstract: A new style floor slab by steel truss and foam cement is introduced in this paper． Due to the use of the new light-weight material foam cement，the floor slab has advantage over the traditional steel truss slab with concrete in further light-weight，thermal and sound insulation properties． In this paper，the short-term stiffness of the floor during the construction phase was studied by the effective moment of inertia method，and a formula was presented． The contribution of precast foam cement layer to the stiffness of the floor cross-section was analysed by an example． The result shows that the contribution of the foam cement layer is a small part of 10%，mainly due to the low strength and small elasticity modulus of the foam cement． Therefore，to simplify the calculation and analysis ，the role of prefabricated foam cement layer can be ignored．