The present invention provides a powder-assembled composite micro-nano fiber and a method for preparing the powder-assembled composite micro-nano fiber. The method includes the following steps: (1) preparing two-dimensional cellulose from a cellulose-raw-material; (2) dispersing the two-dimensional cellulose and a powder material in a solvent to form a mixed suspension; and (3) performing freeze drying on the mixed suspension to obtain the powder-assembled composite micro-nano fiber, wherein a temperature difference between two ends in a vertical direction of the mixed suspension is controlled to be 10-100° C. in a freezing process of the freeze drying. In the present invention, a new non-destructive processing path from primary particles to macro applications is constructed, and rich material platforms and infinite possibilities are provided for basic studies and technical applications. The technology will play a huge role in energy, medical material, environment, protection, catalysis, photoelectricity, food engineering, daily necessity and other vast fields.