Abstract: As the main vertical bearing and horizontal force transmission component in cold-formed thin-walled steel structural residential systems, the cold-formed thin-walled steel composite floor should have good out-of-plane and in-plane mechanical properties to promote the development of light steel structural systems from low-rise to multi-storey. Based on this, the bending stiffness test for normal service stage, bending capacity test and in-plane stiffness test of cold-formed thin-walled steel composite floor were carried out respectively. The test results show that the final out-of-plane failure mode of the composite floor is the buckling failure of C-shaped floor joists at the outermost loading point. The final in-plane failure mode of the composite floor is the self-tapping screw connection failure between the outside C-shaped floor joists and profiled steel sheet. Moreover, the cold-formed thin-walled steel composite floor is a partial shear connection composite floor. The in-plane bearing capacity, stiffness and ductility of the composite floor can be greatly improved by setting floor panel. The bending stiffness and the degree of stiffness degradation of the concrete composite floor are better than those of the gypsum based self-leveling mortar composite floor. Based on mechanical mechanism and failure characteristics of the composite floor, the equivalent bending stiffness formula of T-shaped partial shear connection composite beams is derived by introducing the interface slip modulus between C-shaped floor joists and composite floor slab to calculate the vertical deflection of composite floor. The mechanical analysis model of wide flange deep beam is established and the calculation method of in-plane midspan displacement of composite floor is deduced. The research results provide scientific bases for improving the design and calculation theory of composite floor in cold-formed thin-walled steel multi-storey residential system.