Abstract: The Monte Carlo (MC) global variance reduction (GVR) method based on discrete ordinate (S_N) method was developed. Spent fuel dry storage cask shielding calculation model was established, including forward MC model, S_N model and GVR method model, and the calculation accuracy and efficiency were compared. Numerical results show that the results of GVR method agree well with unbiased MC calculation results and S_N calculation results. The deviation of secondary gamma-ray dose rate between GVR method and S_N method is large for the difference of cross-section libraries. Compared with unbiased MC method, the neutron and secondary gamma-ray transport computational efficiencies of GVR method are improved about 2 orders, and the primary gamma-ray transport computational efficiency is increased about 1-2 orders.