The invention relates to foliar fertilizer for improving wheat powdery mildew resistance and a preparation method thereof. The foliar fertilizer is prepared from sodium bicarbonate, dipotassium hydrogenphosphate, potassium silicate, riboflavin, sucrose, fulvic acid, chitin, phytosterol, trace elements, a chelating agent, a wetting agent, a dispersant and water. The preparation method comprises adding water into a reactor, heating the water to 50-80 DEG C, adding trace elements and a chelating agent into the water, stirring the mixture for a reaction to obtain a solution A, cooling the solution to 40-50 DEG C, adding sodium bicarbonate, dipotassium hydrogenphosphate, potassium silicate, fulvic acid and a wetting agent into the solution, stirring the solution for a reaction to obtain a solution B, cooling the solution to 30-40 DEG C, adding remaining materials into the solution, stirring the materials for a reaction to obtain a solution C, cooling the solution C to the room temperature, filtering the solution and collecting the filtrate to obtain the foliar fertilizer. The foliar fertilizer can improve wheat resistance to powdery mildew, has good permeability, water-absorbing quality, water-retaining property, strong complexation and chelation ability and surface adsorption ability, improves a fertilizer utilization rate and promotes wheat growth.