The invention relates to a low-temperature ball milling preparation method for nano crystal silicon powder with high purity, uniform and controllable grain size and surface passivation. Commercial high-purity silicon powder (the purity is 99.9 percent and the granularity is 50 to 500 meshes) is used as an initial raw material, liquid nitrogen or liquid argon is used as a ball milling medium, and stainless steel balls are used as a grinding medium; and the nano crystal silicon powder with single phase, high purity, grain size of 10 to 100 nanometers and surface passivation can be obtained in alow-temperature ball milling mode by accurately controlling the ball milling time to be between 1 and 48 hours, controlling the ball milling rotational speed to be between 100 and 1,000 revolutions per minute and controlling the ball mass ratio to be 10:1-100:1. The method has the advantages of simple process, low cost and good repeatability; and the prepared nano crystal silicon powder has excellent properties such as high purity, uniform and controllable grain size, surface passivation and the like, and can be widely applied in the fields of photoelectron information and nano technology.