This paper used the OpenSees to simulate an externally prestressed composite beam, by contrasting the experimental results to explore the factors that affect the accuracy of the numerical simulation. The different numerical models are constituted with the different material constitutive models in this paper, with the different section fiber division and other factors. We can consider the method of a part of combination beam section which is connecting with shear rivets. The results show that the different concrete constitutive model and the hardening of the steel constitutive model are the important factors of the accuracy of model, and the Gaussian integration points have 3~5 are bitterly for the simulate mode. This study can provide a theoretical basis for the externally prestressed composite beam’s bending behavior of steel beam section’s numerical simulation with the fiber model.