Steps loading method was used to caculate the whole bending process of a composite beam loaded verticaly in order to accurately analyze the bending performance of composite beams. It has taken into account a variety nonlinear constitutive relations of different materials and based on the layered-step sections transformed method. The displacement of external prestressed steel bars were expressed with rotation angles at anchor points using finite bar elements method. This method is proved reasonable and reliable by comparative analysis of the calculating and the experimental results. It is feasible to analyze the forces and deformations of composite beams which were effcted by nonlinear material factors with layered-step sections transformed method.