The invention discloses a tower-granulated low-nitrogen high-phosphorus urine sulfur radical compound fertilizer and a preparation method thereof, belonging to a fertilizer mixture. The compound fertilizer is characterized by being prepared from the following raw materials in parts by mass: 34-35 parts of urea, 32-33 parts of monoammonium phosphate, 32-33 parts of potassium sulfate and 1-1.5 parts of viscosity reducer, wherein the viscosity reducer is modified lignosulfonate. The invention provides a tower-granulated low-nitrogen high-phosphorus urine sulfur radical compound fertilizer which can meet the nutrition requirements on nitrogen, phosphorus, potassium and sulfur for crop growth and ensures that the product particles are uniform, smooth, mellow and full, small contraction cavities are formed, natural anti-counterfeiting function is realized, caking of the product is prevented, the compressive strength is relatively high and mechanical fertilizing is convenient to carry out; moreover, the invention also provides a preparation method which has advantages of high technical adaptability, high compatibility, energy conservation, small investment and relatively low production cost. The compound fertilizer disclosed by the invention is applicable to various crops such as rice, cotton, corn, peanut, soybean, wheat, rape and the like as well as medium and high-grade cash cropssuch as vegetable, fruit trees, forest trees, flowers, tea and the like. The content of N is less than 20% by weight, and the content of P2O5 is more than or equal to 15% by weight.