Abstract: A powerful simulation tool is required during the decommissioning of research reactor to give the radiation field. Based on the features of complex geometry and large-scale bulk shield, the advantage and disadvantage of deterministic and stochastic methods were discussed in this paper. By analyzing the tendency of the coupled simulation, the coupled Monte Carlo-point kernel transport calculation method was adopted. The radiation field in a large-scale room with bulk shield where the radioactive waste is stored during the decommissioning was calculated. The result shows that under the premise of ensuring the precise, the coupled calculation greatly reduces the computational time comparing with the Monte Carlo calculation.