Abstract: In order to ensure the safety and reliability of reactor pressure vessel (RPV) during the lifetime of nuclear power plant, it is very important to theoretically evaluate the fast neutron fluence rate. In this paper, a method of calculating problem of deep penetration using the adjoint transport theory was proposed. The results of this method were compared with the HBR-2 benchmark data and the deterministic method results. The results show that the calculation results are in good agreement with the measurement results. The relative errors of reaction rates are mostly less than 10% and the maximum relative error is less than 35%. The accuracy of more than 70% of the calculated results is better than that of the deterministic method. It shows that the method proposed in this paper to solve the deep penetration of Monte Carlo is effective and accurate.