In recent years, an emerging technology of Ultra High Performance Concrete (UHPC) has become popular in the construction industry and has been applied in many countries around the world. However, this material technology is still relatively new in Vietnam. The present investigation is a study on enhancement in flexural performance and the effectiveness of UHPC box girder strengthened with pre-stressed tendons, which was adapted from the first project of a high-speed train in Vietnam. A 3D simulation model of a 60 m UHPC pre-stressed box girder was implemented using concrete damage plasticity (CDP) approach. The validity of the proposed model is ensured by comparing the simulation results with experimental data. The parametric studies were then performed using the validated finite element model to analyze the flexural behavior of the 60m UHPC pre-stressed box girder. It was concluded that the developed models can accurately capture the behavior and predicts the load-carrying capacity of the UHPC girder. The present research contributes to the development and application of advanced UHPC concrete technology in Vietnam as well as emphasizes effective designs that significantly reduce self-weight and enhance loading capacity for super long-span girders.