The invention relates to the technical field of aerogel materials and provides a method for preparing flame-retardant thermal insulation type cellulose-based aerogel for solving a problem that traditional cellulose-based aerogel is poor in flame-retardant property. The method comprises the following steps: (1) preparing melamine prepolymerization liquid; (2) mixing a cellulose nanofiber solution and the melamine prepolymerization liquid, and performing ultrasonic dispersion; (3) performing freeze drying to obtain aerogel; and (4) performing drying and crosslinking, thereby obtaining the flame-retardant thermal insulation type cellulose-based aerogel. The preparation method disclosed by the invention is simple and feasible to operate, easily controllable in condition and convenient for large-scale industrial production; and the prepared flame-retardant thermal insulation type cellulose-based aerogel has a self-extinguishing phenomenon after being deviated from fire, is obvious in flame-retardant effect, and still has excellent heat-insulating property and obvious thermal insulation effect.