Problems such as carbonization, corrosion, and durability deterioration would occur to full-iron tailings reinforced concrete (FIT-RC) subjected to long-term use at high temperatures, to find solutions for these problems, it’s of important value to research the flexural performance of FIT-RC beam and its thermomechanical coupling mechanism. This paper proposed a calculation method for designing the flexural capacity and deflection of the normal section of FIT-RC beam, and summarized the high temperature relationship among thermal parameters, thermal stress, and thermal strain of FIT-RC material. In our experiment, four-point bending test was performed on the RC beams and the results showed that the bearing capacity of FIT-RC beam is equivalent to that of conventional reinforced concrete (CRC) beam, the failure modes of the two kinds of RC beams are basically the same. At high temperatures, the similarity of mechanical performance of concrete components with different reduction ratios is related to the similarity of temperature field. With the increase of heating time, the difference in the fire resistance grade of concrete components with different reduction ratios decreases to the minimum value. This study has clarified the high temperature relationship between thermal stress and thermal strain, and verified the feasibility of applying FIT to RC structures.