The invention discloses a composite organic fertilizer of waste oil residue and cake meal and its preparation method, the organic fertilizer is prepared from the following raw materials: by weight, 12-15 parts of the waste oil residue, 30-35 parts of soybean meal, 10-12 parts of medical stone powder, 10-12 parts of zeolite powder, 20-25 parts of oil shale powder, 2-3 parts of sophora flavescens powder, 1-2 parts of quisqualis powder, 10-12 parts of coleus blumei powder, 50-55 parts of manioca leaf powder, 30-35 parts of tea-leaf, 100-120 parts of vinasse, 40-50 parts of vinegar residue, 10-15 parts of edible fungus chaff, 30-35 parts of ammonium dihydrogen phosphate, 16-19 parts of potassium sulfate, 50-55 parts of ammonium bicarbonate, 3-4 parts of a multielement slow release fertilizer and 1-2 parts of an EM (Effective Microorganism) bacteria agent; according to the composite organic fertilizer, the soybean meal, the oil residue, the medical stone powder, the zeolite powder and the like contain large quantities of trace elements and plant chemical components, the contents of related beneficial components in crops can be increased by crop absorption, the nutrition and health care values can be increased, at the same time, the sophora flavescens powder and the quisqualis powder have bactericidal effects, can reduce agricultural crop diseases and insect pests, can reduce chemical pesticide application amount to promote environmental protection.