The invention relates to the technical field of preparation of cellulose aerogel and in particular relates to a preparation method of nano-cellulose aerogel and the nano-cellulose aerogel. The preparation method comprises the following steps: preparing nano-cellulose suspension by utilizing a TEMPO-mediated oxidation method, controlling the oxidation process through a buffer solution, adding a freeze-drying protective additive, performing high-pressure homogenization treatment and freeze drying, and treating, thereby obtaining the nano-cellulose aerogel. The nano-cellulose aerogel prepared bythe preparation method disclosed by the invention has the advantages of high strength, large specific surface area, low density, high porosity, uniform pore size distribution and the like. Moreover, the used freeze-drying protective additive is cheap, readily available, safe and non-toxic. Therefore, the prepared nano-cellulose aerogel does not have any toxicity or cause environmental pollution and is green and environmental-friendly.