Abstract: In large space dose calculation such as nuclear material stockpile, the calculation using Monte Carlo method without variance reduction techniques requires long calculation time and with large error.. A variety of variance reduction techniques such as Weight-Window, Exponential Transform, Forced Collision, DXTRAN Sphere are tested and compared. Then an optimum combination of variance reduction techniques is proposed for the point calculation based on the detector method and for monolithic calculation based on mesh method in large space. Results indicate that the combination of weight windows used forced collision and Exponential Transform with Truncation can greatly improve the efficiency, especially for those points away from the radioactive source.