Bentonite is buffer/backfill material needed in high level waste engineering barrier, and the high plasticity of bentonite affects the quality of buffer/backfill block. In a process of adding water for sample preparation, soil agglomeration and uneven distribution of water content affect the quality of blocks. Compared with traditional spray water method, this experiment explored a new water adding technic, mixing ice powder with bentonite powder (shortly the ice-soil mixing method). Water content is controlled between 5%-30%. In order to evaluate the ice-soil mixing method, the mixing efficiency, mass loss rate, the deviation between actual water content and target water content, the relationship between water content and agglomerated particle size, distribution and contents of agglomerate are measured. Besides, microwave-assisted thawing method is proposed to decrease thawing time. The results show that compared with spray water method, the ice-soil mixed method is less time consuming, less causing adhere to the container, and less causing water content deviate to targets. Moreover, the size and percentage of agglomerate in mixture are also decreased obviously; and water content distribution is more homogeneous; furthermore, microwave-assisted thaw method can significantly accelerate the thawing rate without affecting the amount of agglomerates and water content of specimens. This research provides a reference for the production of buffer/backfill material for High Level Waste (HLW) disposal, and can also be referenced by water content adjustment of other cohesive soil.