The invention discloses a method for direct synthesis of Mn<+>-doped SnO2 nanocrystals with a one-step hydrothermal method, wherein the method comprises the following steps: under stirring, slowly dropwise adding an Sn<4+> salt solution into an NaOH solution, and forming a transparent solution; then dropwise adding an Mn<+> salt solution into the above solution, and forming a suspension; transferring the suspension into a high-pressure reaction kettle, adding distilled water, then sealing, putting into an electric heating drying oven, and at the temperature of 140-200 DEG C, carrying out a reaction for 12-36 h; after the reaction is finished and after the high-pressure reaction kettle is naturally cooled, opening the reaction kettle, taking out a precipitate in a polytetrafluoroethylene container, separating, and respectively washing with distilled water and absolute ethyl alcohol, putting the precipitate in a 60 DEG C vacuum drying oven, drying for 4-6 h, and thus obtaining the product. The Mn<+>-doped SnO2 nanocrystals have the advantages of good morphology and regular dimension; and the method has the advantages of simple instrument and equipment, simple process, easily controlled conditions, and green environmental protection.