The invention discloses a preparation method of a biomass micro-nano fiber aerogel, and belongs to the field of aerogel preparation. The preparation method comprises the following steps: using shrimp shells to prepare purified chitin, obtaining chitin nanofiber through ultrasonic fibrillation treatment, then preparing a gel through dialysis concentration, and finally drying to obtain the biomass micro-nano fiber aerogel. The method is simple in operation and novel, the purified chitin is prepared by using the shrimp shells as raw materials, so that the utilization of waste materials is realized, the waste of resources is reduced, and the prepared aerogel can be self-degraded; moreover, the chitin nanofiber is obtained by using the ultrasonic fibrillation treatment, and the chitin nanofiber is taken as a one-dimensional nano-assembling unit while the chemical structure of alpha-chitin is maintained to have high mechanical performance and form a support with sufficient strength, so that the aerogel is firm in structure and unlikely to fracture, and the tensile and compressive properties of the material are increased.