Introduction: Assessment of body composition provides a more comprehensive evaluation of infant nutritional status than traditional anthropometry . Objectives: This study aimed to characterize body composition patterns in healthy Chinese infants and young children aged 0–36 months and to describe age- and sex-specific developmental trajectories. Methods: 21,059 healthy infants and young children (0–36 months) were recruited from outpatient clinics across 12 provinces and municipalities in China. Anthropometric measurements were combined with bioelectrical impedance analysis (BIA; LEO- 7568) to assess body composition. Percentage of protein, minerals, total body water, muscle mass, skeletal muscle, fat mass, fat-free mass, and regional muscle and water distribution (upper limbs, lower limbs, trunk) were measured. Linear correlation analyses were performed between body composition indicators and anthropometric measures. Results: While no sex difference was observed in mineral percentage (P> 0.05), girls exhibited significantly higher FM% (P < 0.05). Conversely , boys showed higher lower-limb muscle mass and trunk water percentage. Distinct age-related patterns were evident (P < 0.05 for all indices): BMI and FM% peaked initially before gradually declining, whereas other tissue components generally accrued with age. Notably , while length and weight correlated positively with total skeletal muscle and FFM, upper limb muscle mass showed an inverse correlation with growth parameters, suggesting differential tissue accretion rates during early development. Conclusions: Body composition in early childhood follows distinct, sex-specific, and agedependent trajectories. This study establishes a foundation for creating national body composition reference standards in China, facilitating personalized feeding guidance and early metabolic risk screening.