With the increase of computer resources and the application of the global variance reduction (GVR) method, it is a trend to obtain global distribution using the MC method in deep-penetration shielding calculation. The global variance reduction (GVR) technique uses a biased source and weight window to decrease the MC calculation tally error for deep-penetration problems. However, excessive splitting of large weight particles out of the source region is time-consuming for problems with significant fluence rate variation. A new GVR technique, which performs space and energy splitting/roulette based on the importance of the phase space, is proposed in this paper to avoid excessive splitting of large weight particles. The improved GVR technique is applied to HBR-2 benchmark and CAP1400 dose rate distribution calculation. Numerical results show that the new GVR technique shows excellent performance for deep-penetration shielding calculation.