The invention discloses a mixed fertilizer for preventing and treating rice false smut and a preparation method thereof. Raw materials of the mixed fertilizer comprise, by weight, 3-15 parts of potassium polyphosphate, 3-18 parts of white clay, 3-15 parts of straw meal, 5-25 parts of cassia twig, 3-16 parts of fishbone meal, 1-20 parts of sodium humate, 1-18 parts of alga, 1-10 parts of bone meal production leftovers, 10-90 parts of bacillus subtilis, 5-25 parts of rhizoma curcumae, 1-10 parts of radix sophorae flavescentis and 1-18 parts of sea-buckthorn. The preparation method comprises the following steps: crushing white clay, straw meal, cassia twig, fishbone meal, sodium humate, alga, bone meal production leftovers, bacillus subtilis, rhizoma curcumae, radix sophorae flavescentis and sea-buckthorn, sieving through a sieve of 40-80 meshes, uniformly mixing and carrying out thermal insulation at 30-40 DEG C for 5-15 h, mixing and stirring for 10-30 min, adding potassium polyphosphate, uniformly mixing, and granulating so as to obtain the mixed fertilizer. The mixed fertilizer is low-cost and can effectively prevent rice false smut for rice, raise resistance of rice and guarantee stability of rice so as to increase yield of rice.