In this paper the concept of particle size of sub-nanometer wood flour is proposed. The preparation process of sub-nanometer wood flour is analyzed; the equipment structure and principle are discussed in view of a larger practical significance of the sub-nanometer. The theory of grinding sub-nanometer wood flour adopting the methods of circular cutting, grinding and air separation is put forward, in view of the grinding limit, energy consumption laws and physical properties of particle size of wood flour. In the grinding process of sub-nanometer wood flour, the grading equipment and the grinding equipment will be combined into one system, which will filter out the sub-nanometer wood flour constantly which have reached the requirements of mesh number , while the wood flour not meeting the requirements will be put back into the grinding room and processed circularly. In this Process, not only the situation of reunion of fine particles have been reduced and the energy utilization have been improved, but also mesh number of product, uniformity of particle size distribution and consistency for the quality of sub-nanometer wood flour have been ensured, while machining efficiency have been improved.