In order to explain the phenomenon that restrained RC beams subjected to large bending moment do not fail significantly during heating and cooling phases, sectional bending capacity of restrained beams is studied, using the program developed employing FORTRAN. Simulation results show that: (a) bending capacity of RC beams is improved under the axial force generated by axial restraint; (b) with time increasing, N-M envelop diagrams of beams exposed to fire without cooling phase, reduce inward gradually, and especially rapidly in early period, and become more asymmetric; (c) After a period of cooling, N-M envelop diagrams of beams exposed to fire with cooling phase tend to be stable and symmetric.