The invention relates to a method for preparing a network branched silicon nitride single crystal nanostructure with high purity and high yield, and belongs to the technical field of material preparation. The material is the network branched alpha-silicon nitride nanostructure with high purity. The method synthesizes the network branched silicon nitride single crystal nanostructure on a substrate plated with a metal catalyst by by thermally decomposing organic precursor and comprises the following steps of: (1) low-temperature crosslinking curing of polysilazane containing high contents of silicon and nitrogen at a temperature of between 160 and 300DEG C; (2) high-energy ball-milling crushing of amorphous solid after the crosslinking curing in a high wear-resistant utensil; and (3) quick high-temperature thermal decomposition and evaporation of precursor powder obtained after the high-energy ball-milling at protection atmosphere, and precipitation of the precursor powder on the substrate plated with a metal catalyst film. The method has controllable and adjustable components of an evaporation source, simple process and equipment and low cost; and the prepared network branched structure has high yield and high purity, can be used as a reinforcing toughener in a high-performance nanometer composite material, and simultaneously can be used for manufacturing nanoelectronic devices.