The invention provides a powder metallurgy preparation method for a nano-particle reinforced ultra-fine grain metal-matrix composite. According to the method, a metal-matrix grain refining process and a nano-particle dispersing process are carried out in steps. The method comprises the steps that firstly, micro-nano flaky metal-matrix powder is prepared in advance; nano-particles and the flaky metal-matrix powder are stirred and mixed in a stirrer at the high speed under protective atmosphere, and by means of high shear force and pressure generated between stirring blades and a tank body, the nano-particles are uniformly dispersed to the surface of the micro-nano flaky metal-matrix powder; through short-time mechanical ball-milling treatment, the nano metal particles are embedded into the micro-nano flaky metal-matrix powder, so that composite powder of nano-particle reinforced metal is obtained; and compression moulding, sintering and compacting treatment are conducted, so that the ultra-fine grain metal-matrix composite with the nano-particles uniformly dispersed is obtained. By means of the powder metallurgy preparation method, time and energy are saved; cost is low; the application range is wide; and the prepared material is high in comprehensive mechanical performance and has large-scale application potential.