The full replacement of plain concrete with ultra-high toughness cementitious composite (UHTCC) in structural members can obtain the enhanced structural performance over traditional concrete structures because of its prominent advantages such as the markedly ductile deformation capacity after post-cracking and excellent damage tolerance ability. In the current article, aimed at setting up the theoretical formula for the practical use in the bending design of reinforced beam made of UHTCC, the equivalent parameters introduced in rectangular stress block approach are analytically determined. Two existing models in compression for UHTCC, i.e., bilinear model and parabolic line then horizontal line (nonlinear model) are adopted in the derivation of the equivalent parameters. By comparing moment-curvature curve obtained from the experiment with the calculated one, the nonlinear model is verified to be rational and could predict bending capacity with sufficient accuracy. Further comparison between moment-curvature curves calculated according to two compression models shows that the nonlinear model can give a bending response prediction with almost the same accuracy as the bilinear model. Finally, it is suggested that, in the case that nonlinear compression model is adopted for the compressive behavior in UHTCC, the equivalent parameters α=0.8 and β=1.0 could be used to estimate ultimate load bearing capacity of reinforced UHTCC beam.