The invention discloses a preparation method for a nano silicon material. The method comprises the following steps: (1) crude silicon slurry is grinded to obtain micron-sized silicon powder slurry; (2) a carbon material is added to the micron-sized silicon powder slurry for grinding to obtain nano-scale silicon powder slurry; and (3) the nano-scale silicon powder slurry is dried, crushed, sieved,and demagnetized to obtain a nano-silicon material. During the preparation method process, the carbon material is added as a grinding medium, on one hand, granular grinding can be promoted to be finer, and on the other hand, the carbon material replaces the traditional dispersing agent, thereby effectively avoiding the occurrence of hard agglomeration between the nano silicon particles, reducing residual of the dispersing agent, effectively enhancing the material conductivity and further enhancing the electrochemical performance of the material.