To investigate the mechanism of flexural overstrength of cast-in-situ reinforced concrete floor beams incorporating structural spatial restraints, this article takes the spatial position and longitudinal reinforcement ratio of the floor beams in the structure as variables, designs 12 floor beam specimens with upper erection bar not extending into the support, and conducts static tests on the mid-span flexural performance. Based on the experimental method of establishing classical concrete theory, a comparative test of 4 simply supported beams is conducted. The failure mode, bearing capacity, deformation capacity and axial elongation of beam specimens are studied in this article. The results show that the ratio of the bearing capacity of the floor beam specimen to the calculated bearing capacity based on flexural components is 1.57-2.77, which is significantly improved compared to the corresponding rectangular and T-shaped simply supported beam specimens. The degree of overstrength is inversely proportional to the longitudinal reinforcement ratio. Compared with the simply supported beam, the failure mode of the mid-span section of the floor beams do not change significantly due to the axial compression, but the ductility of the floor beam decrease. The axial elongation increases with the increase of the deflection. Regarding the floor beams as compression-flexure components, a method is proposed to determine the corresponding axial force and flexural moment according to the test peak load of the floor beam. The calculation results show that the ratio of the moment of the mid-span section of the floor beams subjected to compression and flexure to the moment subjected to flexure is 1.28 to 2.19. The finite element numerical simulation of the floor beam is carried out in this article. The results show that with the increase of the included floor range of the floor beam section, the axial force tends to be uniform along the beam span.