The invention relates to a potassium-calcium biological composite foliar fertilizer. The potassium-calcium biological composite foliar fertilizer consists of the following components in percentage by weight: 20 to 40 percent of bacteria-containing yeast organic raw material, 24.5 to 39.5 percent of potassium nitrate, 35 to 45 percent of calcium ammonium nitrate and the balance of azone. The invention also provides a preparation method and application of the potassium-calcium biological composite foliar fertilizer. The invention provides a new raw material for the production of the foliar fertilizer, so that the conventional mode of preparing potassium and calcium foliar fertilizers from inorganic raw materials is broken and the new raw material is found for technically improving a composite microorganism fertilizer to the foliar fertilizer. The foliar fertilizer has the characteristics of making waste profitable, lowering the production cost of the foliar fertilizer and improving product quality, along with low cost, high water solubility, high utilization ratio and the like.