The invention provides a method for synthesizing a urea ammonia nitrate (UAN) high-nitrogen slow-release liquid composite fertilizer and relates to a method for synthesizing a composite fertilizer. The method comprises the following steps of: 1, directly utilizing a urea pipeline product (72 percent urea aqueous solution at 90 to 95 DEG C) and an ammonia nitrate pipeline product (94 percent ammonia nitrate aqueous solution at 150 to 160 DEG C) of a chemical fertilizer plant as raw materials, or heating, stirring and dissolving urea and ammonia nitrate to prepare a corresponding aqueous solution to serve as raw materials; 2, directly introducing the two strands of raw materials and water according to the ratio into a reaction tank, and reacting under the conditions of temperature, stirring speed and reaction time; and 3, cooling to obtain the UAN liquid composite fertilizer. The method is simple, process steps and equipment are reduced, and cost is saved; and the reaction condition is mild and pollution is avoided. The UAN liquid composite fertilizer obtained by the method has strong stability, high total nitrogen content (31 to 33 percent), slow-release performance (the nitrogen content is released gradually within 15 days) and proper pH value of 10 percent aqueous solution, and is suitable for foliage dressing.