With the nonlinear finite element method and the program based on ANSYS which is redeveloped by APDL in it, the ultimate bearing capacity of reinforced concrete box girders strengthened with carbon fiber sheets is analyzed in the paper. The material nonlinearities of concrete, steel bars and carbon fiber sheets are taken into account in the three-dimensional finite element models in which the technology of "element birth and death" is used to consider the secondary loads of carbon fiber sheets. Four strengthened RC box girders same with above calculation models are made for experimental researches. The finite element calculation results are in good accord with those of the experimental researches, which verifies that the proposed numerical calculation method for the analysis of the ultimate bearing capacity of reinforced concrete box girders strengthened with carbon fiber sheets is rational and reliable.