The invention belongs to the field of fertilizers, and particularly relates to a compound fertilizer for onions. The compound fertilizer comprises 18 parts of urea, 30 parts of diammonium phosphate, 10 parts of monocalcium phosphate, 30 parts of wormcast, 10 parts of potassium superphosphate, 1 part of molybdenum fertilizer, 1.2 parts of boric fertilizer, 3 parts of ferrous sulfate and 2 parts of magnesium sulfate. In combination with growing needs of the onions, the invention pertinently provides a compound fertilizer formulation; the wormcast is added for providing trace elements needed by the onions; on the premise that soil nutrients are provided, the use of inorganic fertilizers is reduced, so that a negative impact of excessive use of the inorganic fertilizers on acid-base balance of soil is reduced.