A method for preparing mesaconine, comprising the following steps: 1) dissolving aconitine in ethanol, adding sodium hydroxide thereto to hydrolyze, concentrating under reduced pressure, and diluting the residue with water, extracting with dichloromethane to remove impurity alkaloids, acidifying the alkaline aqueous solution with hydrochloric acid, adjusting to pH 11 to 12 by aqua ammonia or a solution of sodium hydroxide in ethanol, concentrating under reduced pressure to dryness, dissolving the residue with dichloromethane-ethanol, filtering, and concentrating the filtrate under reduced pressure to obtain aconine; 2) dissolving aconine in pyridine and reacting with acetic anhydride, concentrating under reduced pressure to obtain a residue, diluting the residue with water, alkalizing with aqua ammonia, extracting with dichloromethane, combining dichloromethane layers, and then drying, concentrating, and separating by silica-gel column chromatography in sequence to prepare 3,14,15-triacetyl aconine; 3) dissolving 3,14,15-triacetyl aconine in glacial acetic acid, adding N-bromosuccinimide thereto, stirring at room temperature, concentrating under reduced pressure to obtain a residue, adding aqua ammonia to the residue, extracting with dichloromethane, combining dichloromethane extracts, drying, and concentrating under reduced pressure to prepare N-desethyl-3,14,15-triacetyl aconine; 4) dissolving N-desethyl-3,14,15-triacetyl aconine in tetrahydrofuran, adding formaldehyde aqueous solution and glacial acetic acid thereto at room temperature, stirring at room temperature, adding NaBH(OAc) 3, continuing to stir, adding aqua ammonia, diluting with water, then extracting with dichloromethane, combining the dichloromethane extracts, and then washing with water, drying, and concentrating under reduced pressure in sequence to prepare 3,14,15-triacetylmesaconine; 5) dissolving 3,14,15-triacetylmesaconine in ethanol solution, adding sodium hydroxide to react with the mesaconine derivative respectively, adjusting to pH 4 to 5 with hydrochloric acid, then adjusting to pH 9 to 12 with aqua ammonia or a solution of sodium hydroxide in ethanol, filtering off insoluble materials, concentrating under reduced pressure, dissolving the residue with dichloromethane-ethanol, filtering with suction, and concentrating the filtrate under reduced pressure to obtain mesaconine 1) dissolving aconitine in ethanol, adding sodium hydroxide thereto to hydrolyze, concentrating under reduced pressure, and diluting the residue with water, extracting with dichloromethane to remove impurity alkaloids, acidifying the alkaline aqueous solution with hydrochloric acid, adjusting to pH 11 to 12 by aqua ammonia or a solution of sodium hydroxide in ethanol, concentrating under reduced pressure to dryness, dissolving the residue with dichloromethane-ethanol, filtering, and concentrating the filtrate under reduced pressure to obtain aconine; 2) dissolving aconine in pyridine and reacting with acetic anhydride, concentrating under reduced pressure to obtain a residue, diluting the residue with water, alkalizing with aqua ammonia, extracting with dichloromethane, combining dichloromethane layers, and then drying, concentrating, and separating by silica-gel column chromatography in sequence to prepare 3,14,15-triacetyl aconine; 3) dissolving 3,14,15-triacetyl aconine in glacial acetic acid, adding N-bromosuccinimide thereto, stirring at room temperature, concentrating under reduced pressure to obtain a residue, adding aqua ammonia to the residue, extracting with dichloromethane, combining dichloromethane extracts, drying, and concentrating under reduced pressure to prepare N-desethyl-3,14,15-triacetyl aconine; 4) dissolving N-desethyl-3,14,15-triacetyl aconine in tetrahydrofuran, adding formaldehyde aqueous solution and glacial acetic acid thereto at room temperature, stirring at room temperature, adding NaBH(OAc) 3, continuing to stir, adding aqua ammonia, diluting with water, then extracting with dichloromethane, combining the dichloromethane extracts, and then washing with water, drying, and concentrating under reduced pressure in sequence to prepare 3,14,15-triacetylmesaconine; 5) dissolving 3,14,15-triacetylmesaconine in ethanol solution, adding sodium hydroxide to react with the mesaconine derivative respectively, adjusting to pH 4 to 5 with hydrochloric acid, then adjusting to pH 9 to 12 with aqua ammonia or a solution of sodium hydroxide in ethanol, filtering off insoluble materials, concentrating under reduced pressure, dissolving the residue with dichloromethane-ethanol, filtering with suction, and concentrating the filtrate under reduced pressure to obtain mesaconine