Objective: Sialic acid (SA) in human milk mainly exists in oligosaccharide bound state， accounting for about 70% of the total sialic acid， whereas in most infant formulas， most SA is in a protein-bound form. However， the differences in the effects of different binding forms of SA on short- and long-term outcomes and their mechanisms remain unclear. This study aimed to explore the relationship between different forms of SA in breast milk at various stages and infant growth and development within the first six months， providing a scientific basis for improving maternal and infant nutrition in early life. Methods: Based on the China maternal and infant nutrition and health birth cohort study， 29 mother-infant pairs who were exclusively breastfed within six months postpartum were selected for the study. Breast milk was collected at 0-7 days， 8-15 days， 1 month， and 2-6 months postpartum. Total SA content， oligosaccharide-bound SA content， protein-bound SA content， and lipid-bound SA content in colostrum， transitional milk， and mature milk were measured using high performance anion-exchange chromatography with pulsed amperometric detection. Infant growth and body composition indicators from 0 to 6 months were also measured. The associations between SA content in breast milk and infant growth and body composition indicators and their changes were analyzed. Results: Spearman partial correlation and linear regression analysis showed that total SA and oligosaccharide-bound SA in breast milk were negatively correlated with infant weight and head circumference (P<0.05). The head circumference of infants with high content of total SA and oligosaccharide-bound SA in breast milk was significantly lower than that of infants with low content(P<0.05). The head circumference at 1 month of age in infants from the high total SA group in colostrum was lower than that in infants from the low total SA group. The head circumference of infants in the high oligosaccharide-bound SA group in colostrum from January to May was lower than that in the low oligosaccharide-bound SA group. The head circumference of infants with high total SA content in transitional milk was lower than that of infants with low total SA content from 1 to 4 months， but there was no significant difference(P＞0.05). Conclusion: In order to meet the needs of neonatal brain SA， when the neonatal head circumference is small， breast milk could adjust adaptively and increase the total SA content. SA was negatively correlated with infant head circumference and weight， and its level could reflect the needs of infant growth and development， and provide a scientific basis for improving infant nutrition.