Abstract: In order to solve the problem that the bottom plate of a common concrete composite slab is easy to crack and a lot of vertical supports are needed in its construction, a kind of oblique rib type free support tool-truss composite slab (FSTCS) was developed. Three specimens with the same span and different structural measures (overlap, welding and wire mesh) were loaded by standard weights, it was clear that the structural measures had no significant influence on the overall flexural performance of composite slabs, and overlap was a simple and efficient measure. Through the test and numerical study of three specimens with the same structural measures and different spans, the working mechanism of FSTCS was proved, and the theoretical method for calculating the flexural stiffness and deflection was established based on the superposition principle. The results show that: compared with the measured values, the theoretical method and numerical model have high precision, and the minimum determination coefficients are 0.994 and 0.988, respectively. There is no obvious synergistic effect between the upper steel tube and the bottom slab, the contribution of the truss between them to the flexural stiffness can be ignored when the span is small, and the contribution of specimens with a span of 3.4 m and of 4.3 m to the total flexural stiffness before and after cracking is 12.1%, 22.0% and 15.0%, 26.7%, respectively. The flexural performance of FSTCS is characterized by two stages with cracking as the boundary, after cracking, the overall flexural stiffness decreases rapidly first and then slowly, and the contribution of steel tube to flexural stiffness of composite slab is always the largest. The research results provide a reference and calculation method for the application of the FSTCS.