Abstract: Ultra High Performance Concrete (UHPC) was used to construct prefabricated formwork of 10 mm thick. The construction phase loading test was carried out to evaluate the feasibility of the UHPC formwork as a formwork for cast-in-place concrete slab. Six simply supported unidirectional slabs and a two-span continuous unidirectional slab were manufactured and tested, for which the prefabricated UHPC formwork was used as the bottom formwork. The results show that when the UHPC formwork of 10 mm thick is used as the building formwork, the supporting distance under the formwork can be 0.5 m, and uniformly distributed load of up to 6 kN/m² can be applied on the UHPC formwork in the elastic state, which is approximately 1.7 times the design value of the construction uniformly distributed load. The interface between the UHPC formwork and post-cast-inplace concrete does not exhibit a visible bond-slip phenomenon when the composite slab is destroyed. The flexural strength of the slab is approximately two times that of a RC slab with the same cross-section and reinforcement. A formula for calculating the cross-sectional flexural strength of unidirectional slabs is established,which considers the tensile force in the prefabricated UHPC formwork and concrete in the tensile region. The strengths predicted by the formula show good agreement with the experimental results.