OBJECTIVE: To characterize the longitudinal trajectories of body-mass-index-for-age Z-score(BAZ) from birth to 7-9 years and to explore their association with elevated blood pressure(EBP) in childhood. METHODS: Based on the TAWS prospective cohort study in China, children living in Taicang were followed from 2013 to 2024. Length/height and weight of children were measured at 1, 3, 6, 8 months, and 1, 1.5, 2, 2.5, 3, 4, 5, 6, 7-9 years of age. Blood pressure was measured in 2024 when children were 7-9 years old, and EBP was defined according to Chinese reference of screening for EBP among children. BAZ was calculated based on the World Health Organization child growth standards. Group-based trajectory modeling(GBTM) was applied to identify BAZ trajectories. Multivariate logistic regression was used to assess the association between BAZ trajectories groups and EBP, adjusting for potential confounders. RESULTS: Among the 1, 274 children included in this study, the prevalence of EBP was 20.6%. GBTM identified three distinct BAZ trajectories: low-decreasing(34.5%), moderate-stable(48.3%), and high-increasing(17.2%). After adjustment for child's gender, initial BAZ, and age at the last follow-up, children in the high-increasing BAZ group had an increased risk of EBP(OR=1.55, 95%CI 1.07-2.23, P=0.02) compared to those in the moderate-stable BAZ group, while the low-decreasing BAZ group showed no statistically significant association with EBP(OR=0.77, 95%CI 0.55-1.06, P=0.11). From 2.5 years old onwards, BAZ values in children with EBP were consistently higher than those with normal blood pressure(P<0.05). CONCLUSION: The high-increasing BAZ trajectory from birth to nine years is associated with an increased risk of EBP in children aged 7-9 years. The age of 2.5 years may represent a critical starting point for interventions aimed at controlling BAZ rise to mitigate future EBP.