Abstract: To determine the flexural capacity of corroded reinforced concrete (RC) beams after high temperature, the simplified section analysis method was used to calculate the characteristic strain of corroded steel bars after high temperature, and the stress state of steel bars was judged by it; then, four failure modes were proposed and three bending failure limits were defined. The failure mode of the corroded RC beams after high temperature was judged by the characteristic strain, and the flexural capacity of the beam was calculated accordingly; finally, the bending capacities of 10 test beams designed in this paper and 40 concrete beams collected in literature were calculated by using this method. The results show that with the increase of corrosion rate and temperature, the flexural failure mode of the normal cross-section of an initially under-reinforced RC beam changes in the order of “mode A”, “mode B”, “mode C” and “mode D”; according to the theoretical strain of corroded steel bars after high temperature, the failure modes of 50 concrete beams calculated in this paper are mainly “mode B” and “mode C”, and the minimum bound corrosion rate of 10 test beams from “mode B” to “mode C” is 22%. The average ratio of calculated values to test values of test beams and literature beams is 0.913 and 0.957, respectively. The accuracy of the calculation method is verified, which can provide a basis for the reinforcement and repair of corroded RC beams after high temperature.