Background: Human milk (HM) is a proven optimal food for preterm infants. However, there is uncertainty regarding the effects of different proportions of HM of the total enteral intake on health outcomes in preterm infants. Therefore, we conducted a systematic review of studies examining the effects of different proportions of HM of the total enteral intake on health outcomes in preterm infants. Methods: We conducted a literature search in the Web of Science, PubMed, and Scopus databases. The methodological quality of the included articles and the certainty of evidence were assessed according to the Newcastle-Ottawa Scale and the Grading of Recommendations, Assessment, Development, and Evaluation (GRADE) tool, respectively. Results: Twelve studies were included in the review. Among the clinical findings, the effect of different proportions of HM of the total enteral feeding on health outcomes in preterm infants was divided into six primary outcomes: physical growth, length of stay, morbidity of any disease, all-cause mortality, feeding-related outcomes, and other health outcomes. The studies presented a high risk of bias for most of the domains. The certainty of the evidence was considered low or very low. Conclusions: The findings reiterated that greater proportions of HM positively affect the health outcomes of preterm infants. Overall, when the HM accounts for at least 20% of the total enteral intake, it has an effect on health outcomes in preterm infants. If the proportion of HM reaches 50%, the incidence and severity of necrotizing enterocolitis, as well as the time to reach enteral feeds, will be reduced. Increasing the proportion of HM in enteral feeding should be considered a priority in the feeding strategy for preterm infants in clinical practice.