The present study examined additive composition as a longitudinal predictor for symbolic numerical magnitude comparison. Two hundred and twenty-four Chinese kindergarteners (M = 63.76 months; 103 boys, 121 girls) participated in this study across three waves of cognitive assessments. Cross-lagged panel analyses showed that additive composition predicted symbolic numerical magnitude comparison longitudinally but not vice versa. The cross-lagged effects remained significant beyond the influence of general intelligence, working memory, nonsymbolic magnitude comparison, and counting abilities. Theoretically, this study suggests that additive composition is a key factor underlying the exact representational system of numbers. It also entails educational implications that understanding the quantitative relations between numbers should be the focus of learning and teaching in early childhood mathematics education.