The invention discloses a compounded organic fertilizer and a preparation method therefor. The compounded organic fertilizer is prepared from the following raw materials in parts by weight: 30-50 parts of livestock and poultry manure, 10-15 parts of industrial and agricultural wastes, 3-5 parts of humic acid, 3-5 parts of activating powder, 4-6 parts of magnetized water straws, 2-4 parts of peat soil, 0.3-0.5 part of middling, 5-15 parts of soybean meal, 10-15 parts of rice hull powder, 1-5 parts of strain powder, 3-5 parts of urea, 4-6 parts of polyacrylamide, 0.1-5 parts of sodium glutamate, 0.1-5 parts of sodium alginate, 10-14 parts of monoammonium phosphate and 10-16 parts of potassium sulfate. According to the preparation method, the compounded organic fertilizer is prepared through firstly, preparing crushed materials from part of the raw materials, then, adding the remaining raw materials into the crushed materials, carrying out uniform stirring, then, carry out physical fermentation for 20 to 30 days, and then, carrying out drying and granulating. The compounded organic fertilizer is rich in organic substances and nutrients required for crop growth, is easy to decompose, is used for amending soil and can be used for improving the quality of crops and increasing the yield of the crops.