Objective: To understand the development trend of blood group antibody by detecting the titer of ABO blood group IgM antibody of infants and young children within 18 months of age in Nanjing. Methods: Samples of 18-month-old hospitalized infants and young children were collected and blood type was detected by microcolumn gel method. A total of 1250 non-AB samples were obtained, including 454 cases of A type, 410 cases of B type, and 386 cases of O type. The titer of IgM blood group antibody in plasma was measured and the aggregation degree was counted. Results: The number of infants with anti-A and anti-B titers of 0 within 30 days of age accounted for 33.88% and 47.46% of this age group respectively; between 2 and 6 months, the number of infants with titers of 0 and low titers gradually decreased; at 7 months of age, the titers of anti-A antibodies in infants and children were ≥2; at 13 months of age, the titers of anti-B antibodies in infants and children were ≥2. The result of the agglutination score suggests that the strength of the ABO blood group IgM antibody gradually increases with the age of the month. Conclusion: Infant blood group IgM anti-A development is basically completed at 7 months of age, IgM anti-B development is basically completed at 13 months of age, and IgM blood group antibody development has an early trend.