In order to calculate prestress increment of external tendon in externally prestressed beam and compute structure deformation considering stiffness of externally prestressed tendons under the situation of vertical load, using displacement model of element which is expressed by dof of beam and the slip at deviation block of externally prestressed tendons, the finite element stiffness equation of externally prestressed beam has been derived based on large-deformation theory. Then this numerical solution is used to calculate the prestress increment of externally prestressed tendons and the structure deformation of externally prestressed beam under the vertical load. Compared by laboratory test results of externally prestressed beam, the finite element solution is good consistent with experimental values, therefore using this finite element solution to analyze the flexural behavior of externally prestressed beam is credible. The results show that this numerical solution based on large-deformation theory can compute stress and deformation of structure very accurately under the situation of normal using state. So this process is provided a new idea in design of externally prestressed beam.