Steel-FRP composite bar (SFCB) has the advantages of both steel bars and fiber-reinforced polymer (FRP), and its excellent corrosion resistance can be combined with coral concrete, which has a good application prospect in the development of islands in the distant sea. In order to study the flexural mechanical properties of coral concrete unidirectional slabs reinforced by SFCB, six unidirectional plate specimens were subjected to static monotonic flexural loading test with varying parameters such as the steel bar type, longitudinal reinforcement ratio and concrete type. The failure process and morphology of the unidirectional slab under force were observed, the flexural moment-deflection curves and material strain distribution of the entire force process were obtained, and the influence of variation parameter on the flexural performance of the unidirectional slabs was analyzed. The results show that the failure mode of the SFCB specimen is similar to that of the ordinary steel specimen, which belongs to the flexural failure of normal cross-section, and has stronger bending stiffness after concrete cracking, the crack development is effectively limited. Compared with odinary steel specimen, the ultimate flexural moment is increased by 8.4%, but the ductility is reduced by 12.7%. The bending stiffness and bearing capacity of SFCB specimens decrease with the decrease of reinforcement ratio, but the ductility is improved, with a maximum increase of 71.2%. The surface porousness, micro-pumping effect and brittleness of coral aggregate lead to higher bearing capacity and poor deformation ability of SFCB coral concrete slab specimen, but its failure form is similar to that of ordinary concrete slab specimen. Finally, based on the assumption of flat cross-section, the formulas for calculating the ultimate flexural moment of concrete unidirectional slabs reinforced by SFCB are derived, and the calculated values are in good agreement with the test values.