Experiment on an externally prestressed ultra high performance concrete (UHPC) box girder subjected to symmetrical concentric loads is carried out. The behavior of the test girder is investigated, including the load-deflection curve at mid-span, cracking pattern, strain distribution, and failure mode. The mid-span nominal moment capacity of the girder is analyzed. Test results show that the anti-cracking capacity of UHPC box girder is better than that of a normal prestressed concrete (PC) girder. The tensile property of UHPC should be taken into account in the process of calculating mid-span nominal moment capacity of the externally prestressed UHPC box girder. Finite element (FE) model analysis results agree well with the test results. The influencing parameter, concrete grades, was studied numerically to compare the flexural behavior of UHPC girder with PC girder. The cracking moment and ultimate load calculation method is proposed, which can meet precision for engineering practice and can be a reference method for design calculation of a prestressed UHPC box girder.