The invention provides a magnetized carbon-based fertilizer and a preparation method thereof. The magnetized carbon-based fertilizer comprises, by mass, 25-40 parts of agriculture and forestry wastes, 20-25 parts of fly ash, 15-18 parts of ammonium sulfate, 9-12 parts of urea, 6-8 parts of potassium sulfate, 1-2 parts of monoammonium phosphate, 9-13 parts of expansive soil and 1-2 parts of an adhesive. The preparation method of the fertilizer comprises the following steps: mixing above raw materials, deodorizing the obtained mixture with a composite microbial starter, carrying out piling fermentation on the mixture, turning the obtained pile and overturning to completely keep nutrients, carrying out high temperature fermentation to kill grass seeds with diseases and pests, decomposing the mixture to eliminate pesticide residual, drying and crushing the decomposed mixture, screening the crushed mixture to remove impurities, magnetizing the screened mixture, spraying a bacterial liquid, drying the sprayed mixture, and granulating the dried mixture. The preparation method has the advantages of no seasonal restrictions, low management cost, full utilization of resources, changing of wastes into valuables, realization of no peculiar smell of the produced organic fertilizer, harmless treatment, short organic fertilizer production time, simplicity in operation, convenience in management, substantial improvement of the earth temperature, prolongation of the effective period, soil improvement, and promotion of early growth and quick tillering of crops.