The invention belongs to the technical field of functional food and food for special medical purposes and specifically relates to a breast milk nutritious supplementary agent for a premature infant or an infant with extremely low birth weight. The breast milk nutritious supplementary agent is used for supplementing nutrients required by the infant growth and development. The breast milk nutritious supplementary agent provided by the invention comprises the following components by weight percent: 10-14% of fresh milk (dry matter), 20-30% of whey protein powder, 2-4% of compound mineral substances, 1-3% of compound vitamins, 0.1-1% of iron compounds, 0.5-2% of breast milk nutrients and the balance of maltodextrin, or comprises the following components by weight percent: 5-15% of skim milk powder, 20-30% of whey protein powder, 2-4% of compound mineral substances, 1-3% of compound vitamins, 0.1-1% of iron compounds, 0.5-2% of breast milk nutrients and the balance of solid glucose syrup. The breast milk nutritious supplementary agent prepared in proportion according to the invention can guarantee the effects of all nutrients and all the nutrients have synergic effects, so that the healthy growth of the premature infant can be more effectively promoted.